Unable to check for updates...Make sure you are connected to the Internet.

This method may seem unnecessary but sometimes the Router Restart clears Network issues from ISP to Apple Servers

Shutdown Router. Shutdown computer and disconnect all External Drives. Restart Router. Connect to Router via Ethernet Cable. If via Wifi - Disconnect all other Devices using the Wifi - want the Maximum Bandwidth only for this computer. Restart computer in  Safe Mode. It does a Repair Disk, Clear System Cache files and only load Apple Software, extensions and Fonts. Will load slowly - Normal. Once Logged in - open Apple Apps Store and  Get the Full Version of Big Sur. It will take some time and several pauses will occur - normal. Countdown clock may say 15 minutes but in Real time may be 30 minutes. Same goes for clock at 10 minutes could be 20 minutes in Real time. Eventually , Full Installer will download and a New Application in Applications Folder will present called Install Big Sur. Stop there and perform a Time Machine Backup. If desired - have an External Drive - Copy & Paste the Install Big Sur Application to external drive for future usage - making a Bootable Installer. Now run the Installer.

Have found this method more reliable than the Software Update which is very iffy at best

Hank Testa1 wrote:

Have been having this problem since seeing there is an update to Big Sur. I get the same response from the "About this Mac...Software Update" or by going directly to the App Store. In App Store I see "macOS Big Sur" but when I click on VIEW I get the same "Unable to check for updates" response. Other apps download/install OK. ?????

I have a 2020 Mini M1. Big Sur 11.4.

Did the Wi-Fi diagnostics with no meaningful result other than while diagnostics was monitoring, the connection dropped. Diagnostic report was of no help. I've reset the modem & Apple Airport Extreme & rebooted the computer several times.

Is there something about the Big Sur 11.5 update that is incompatible with my Mini?

Any suggestions are appreciated.

I would simply try the point update (~ 3.84GB) at later time/date if there are server issues beyond your control

or

from the terminal.app the full installer (~13GB) copy & paste:

sudo softwareupdate --fetch-full-installer --full-installer-version 11.5

Start up a Mac with Apple silicon in safe mode

1. On your Mac, choose Apple menu  > Shut Down.
2. After your Mac shuts down, wait 10 seconds.
3. Press and hold the power button on your Mac until the startup options window appears showing available startup disks and Options.
4. Select a startup disk.
5. Press and hold the Shift key, click Continue in Safe Mode, then release the Shift key.

Have seen at least two others with similar issue and theirs was a piece of Malware playing up and blocked the check for updates.

Are there any AntiVirus, Disk Cleaner, Optimizers, Un-installers, etc installed which should be removed as per Developers Instructions. They are useless, unneeded, cause havoc and interfere with the normal operation of the OS and may even Corrupt the OS requiring a Reinstallation. The Built-in Security of Big Sur is all that is required.

There are no known Viruses in the wild that self replicate and affect macOS. There are Malware and Adware that does affect macOS and are often times downloaded as part of an Application from Third Party UnTrusted Site and get installed along with the Application.

Suggest downloading from a Trusted Developer and Respected ASC Contributor the application Malwarebytes for Mac. It is free or paid for added features. Run the Application and it should remove the malware / adware. Once done, restart computer and test.

Start up your computer in safe mode Pertains to M1 CPU Computer

1. Choose Apple menu  > Shut Down.
2. Press and hold the power button on your Mac until you see “Loading startup options.”
3. Select a volume.
4. Press and hold the Shift key, then click Continue in safe mode.
5. The computer restarts automatically.

Try this: boot into Safe Mode according to Start up your Mac in safe mode - Apple Support and try downloading the update and install from there.

NOTE: Safe Mode boot can take up to 10 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

Get RID of the invasive, useless PCMatic software as per the Developers Instructions and once that is conformed, come back and report the computer condition

Are there any AntiVirus, Disk Cleaner, Optimizers, Un-installers, etc installed which should be removed as per Developers Instructions. They are useless, unneeded, cause havoc and interfere with the normal operation of the OS and may even Corrupt the OS requiring a Reinstallation. The Built-in Security of Big Sur is all that is required.

Running behind a VPN? It might be intercepting the packets to / from Apple Servers and making the issue you are experiencing.

Etrecheck will give a good Snap Shot of the Hardware and Software. It will offer insights. It will not Hurt. If going this route - really do ask to run the Application with Full Disk Access and post back the Full Report.