iMac can’t erase volume because kernel 0 is using it!

Let's try this (this will erase all data):

1. Shut down the Mac.
2. Turn on the Mac, and immediately press and hold Option + Command + R until you see a spinning globe.
3. Once you are in macOS Recovery, verify you are connected to WiFi in the top right corner.
4. Go to Disk Utility.
5. Select "View" in the toolbar —> "Show All Devices".
6. Select your main hard drive or SSD. This will be the top, non indented hard drive. 
7. Click Erase, and input the following exactly: (If you cannot select erase on the main hard drive, erase the "Container" or “Macintosh HD”.)
8. Name: Macintosh HD
9. Format: APFS
10. Scheme: GUID Partition Map (if shown)
11. Click Erase. If “Erase Volume Group” is shown, use that instead. 
12. Quit Disk Utility
13. Go to the reinstall macOS app.
14. Follow the prompts, and install on Macintosh HD.

Cheers,

Jack

Going back to erasing the Mac, did you try Internet Recovery?

> they can’t repair it unless I can magically poof the old ram back

So you have 3rd party RAM installed and Apple won't touch it, right? If yes, put the old RAM back. Should the new RAM be OK??

Or DIY. Can you make and boot from a USB macOS installer flash drive?

https://support.apple.com/en-us/HT201372

Download and run Etrecheck. Etrecheck is a diagnostic tool that was developed by one of the most respected users here in the ASC and recommended by Apple Support  to provide a snapshot of the system and help identify the more obvious culprits that can adversely affect a Mac's performance.

IMPORTANT:

Before running Etrecheck assign Full Disk Access to Etrecheck in the Etrecheck's Privacy preference pane so that it can get additional information from the Console and log files for the report:

Also click and read the About info to further permit full disk access.

Copy the report

and use the Additional Text button to include the report in your reply.

Then we can examine the report and see if we can determine what might be causing the problem.

Honestly, there may be a way to improve performance without erasing. If so, we need an EtreCheck report.  EtreCheck is a useful app to diagnose and identify issues, both hardware and software. It is trusted by many users. Please follow the steps outlined below:

1. Navigate to: https://etrecheck.com and get the free version of EtreCheckPro.
2. Download it. It is a free and trusted app. It is often recommended here on Apple Support Communities. 
3. Open EtreCheck, and before running the report, be sure to check “Allow full Drive Access” at the bottom of the EtreCheckPro window.
4. Run the report, and put it in a post, as seen here: https://discussions.apple.com/docs/DOC-250000211. We will then examine the report for issues in your Mac.

Cheers,

Jack

This may be possible now, but earlier the OP said "No It can’t just be a little speed up, it was an update that made it lag so bad I could barely even use safari and I can’t go and get etre check because I can’t even boot the operating system" about EtreCheck.

Cheers,

Jack

Hi Insanelogic,

Are you trying to erase your internal drive (Macintosh HD) or an external disk?

Cheers,

Jack

What year is your Mac and current macOS version?

Good luck!