Is there a way to access Single User mode through terminal? My computer won’t boot (command+S).

You can do many of the things in Terminal that you can in SU modem but disk Repair may not work very well do to open files.

Repair a storage device in Disk Utility on Mac

Disk Utility can fix certain disk problems—for example, multiple apps quit unexpectedly, a file is corrupted, an external device doesn’t work properly, or your computer won’t start up. Disk Utility can’t detect or repair all problems that a disk may have.

If you run First Aid on a disk, Disk Utility checks the partition maps on the disk and performs some additional checks, and then checks each volume. If you run First Aid on a volume, Disk Utility verifies all the contents of that volume only.

1. In the Disk Utility app  on your Mac, choose View > Show All Devices.
2. Note: If you’re checking your startup disk or startup volume, restart your computer in macOS Recovery, select Disk Utility in the macOS Utilities window, then click Continue. If you check your startup volume (Macintosh HD), make sure you also check your data volume (Macintosh HD - Data).
3. 
   
4. In the sidebar, select a disk or volume, then click the First Aid button .
5. If Disk Utility tells you the disk is about to fail, back up your data and replace the disk—you can’t repair it. Otherwise, continue to the next step.
6. Click Run, then click Continue.
7. If Disk Utility reports that the disk appears to be OK or has been repaired, you’re done. You can click Show Details to see more information about the repairs. Otherwise, you may need to do one of the following.

• • If Disk Utility reports “overlapped extent allocation” errors, two or more files occupy the same space on your disk, and at least one of them is likely to be corrupted. You need to check each file in the list of affected files. Most of the files in the list have aliases in a DamagedFiles folder at the top level of your disk.
    • If you can replace a file or re-create it, delete it.
    • If it contains information you need, open it and examine its data to make sure it hasn’t been corrupted.
  • If Disk Utility can’t repair your disk, or you receive a report that the First Aid process failed, try to repair the disk or partition again. If that doesn’t work, back up as much of your data as possible, reformat the disk, reinstall macOS, then restore your backed-up data.

If your Mac has a Fusion Drive and you see a flashing question mark or alert, see the troubleshooting section of the Apple Support article About Fusion Drive, a storage option for some Mac computers.

If you continue to have problems with your disk or it can’t be repaired, it may be physically damaged and need to be replaced. For information about servicing your Mac, see Find out how to service or repair your Mac.

AhliasTPCC1 wrote:

my Internal HDD S.M.A.R.T. Status is “Failing”. I ran first aid on both the disk and the volume and it said everything was ok. But the S.M.A.R.T. Status is still “Failing”

I'd swap that storage device.

If it's a hard disk drive, I'd also swap for an SSD if I planned to keep this Mac for a few years. SSDs are vastly faster. And if the Mac supports an upgrade—newer models don't—I'd add more memory while the box is open.

Even in the unusual event that the SMART data is mis-detecting failures or that this detected error is benign or otherwise isolated, I'd still assume the storage is failing. Hard disk drives can and do fail. (As can SSDs, but those usually fail hard.)

One hard disk drive failure pattern is degrading performance and corruptions and crashes. Eventually, that pattern usually transitions over into increasingly-visible storage errors when the data written cannot be read back, or the data is corrupted when written, and eventually outright failure.

Repairs and reconstruction doesn't fix these cases, and the I/O activity involved in verification and repair attempts can serve to push the storage further into failure.

Get a backup if not already, and expect to have that storage device replaced. Local rule of thumb is backups first and second, and maybe poking around third or fourth, prior to the swap. Use whatever remaining I/O might be available to preserve data, if there are not current backups.

Terminal and the command line will provide little benefit past what the higher-level tools can already achieve.

SMART tries to be predictive—giving some warning of impending failure, as is this case—but is also not reliably predictive of failures, too. SMART has empirically missed many hard failures.

If you want to poke at the SMART data and see how bad this device is—given your inferred proclivities here, I'd expect that's going to be of interest, DriveDX can be useful. But ensure you have complete and current backups first, should this storage device be following the usual pattern of degrading toward failure.

And FWIW, among those Mac models that can be repaired or upgraded, some are easier to repair or upgrade than others. The iMac 21.5" series isn't particularly "fun" to repair or to upgrade, though it's possible.

Given the 2011 vintage here, this iMac is probably better replaced with a new or new-to-you Mac

If you do decide to buy new, the current low-end 21.5" iMac configuration is good for mail and FaceTime and looking at photos and such, but the entry-level 8 GB memory with a 5400 RPM hard disk drive (slow!) is going to be very slow for many tasks. The mid- and higher-spec iMac configurations include SSD, and are vastly faster.

Backups? Booting Recovery and using Disk Utility is the usual path for creating an external backup of an internal storage device.

If you can't repair or replace right now, using Disk Utility from Recovery to copy your internal storage to external and then booting from the fastest available external storage can provide a work-around for a failing (or failed) internal storage device.

AhliasTPCC1 wrote:

my Internal HDD S.M.A.R.T. Status is “Failing”. I ran first aid on both the disk and the volume and it said everything was ok. But the S.M.A.R.T. Status is still “Failing”

macOS usage of the SMART Status is "Failing" is only triggered if one of the pre-failure SMART attributes on the drive has run out. This condition is determined by the manufacturer of the drive. Most drives never get to this condition because a drive usually becomes unusable long before this happens. If macOS/Disk Utility is showing "SMART Status is Failing", then most likely the drive is bad and needs to be replaced immediately. Definitely make sure you have a good backup or you risk losing your data.

Edit: Disk Utility First Aid only checks the health of the file system and partition table. Except for the "SMART status" indicator Disk Utility does not actually monitor the health of a drive. macOS does not actually monitor the health of the drive or it would warn you long before the "SMART Status" indicator is triggered as "Failing". Not all types of drive failures will cause file system issues.

I'd Clone to an external drive & boot from that, which exact Mac do you have?

https://eshop.macsales.com/shop/firewire/1394/USB/EliteAL/eSATA_FW800_FW400_USB

Carbon Copy Cloner...

http://www.bombich.com/software/ccc.html