Mac crashes and restarts with the following error. Note: I do not have an external or separate monitor. panic(cpu 2 caller 0xffffff8011a8915a): Sleep transition timed out after 180 seconds

You need to post the complete panic log to the Additional Text area, here (document icon).

Panic logs are located in /Library/Logs/DiagnosticReports and start with "Kernel."

It is possible that the panic log wasn't written to disk, so you may not find one. Every other log in that folder is of no use whatsoever.

Try a few things that might help, but likely won't (and they don't hurt).

1. SMC Reset (WiFi issue).
2. NVRAM Reset (fix possible extremely rare source of kernel panics).
3. Erase the drive and reinstall macOS. You can migrate data, but don't migrate apps until you check to see if you are having any problems. But, you don't have anything installed that would cause a kernel panic, so this might just help with the other problems.

Forgot to ask. Do you have any other hardware attached to the Mac when it panics?

I have seen where it seems an external drive has caused panics, but it is rare. Certainly worth disconnecting to see if it is that. The panic might have occurred in the input/output framework, so it is possible that it is the drive. Maybe a change to IOKit in Big Sur has a bug or just doesn't like your drive.

You don't have any third-party kernel extensions installed, so that tends to make a hardware fault likely.

However, it could have just been a one-off error that wasn't handled (or an obscure bug in Apple's kexts).

Did it just happen the one time, or does it happen often?