How do you fix, “The supplied iCloud account was unable to unlock this volume”?

Hello Brady-B.

Thank you for using Apple Support Communities. This issue might be related to resetting your admin password on your Mac. Try starting up in safe mode to see if you're able to change your password properly:

How to use safe mode on your Mac - Apple Support

On an Intel processor Mac:

1. Turn on or restart your Mac, then immediately press and hold the Shift key as your Mac starts up.
2. Release the key when you see the login window, then log in to your Mac. 
3. You might be asked to log in again. On either the first or second login window, you should see ”Safe Boot” in the upper-right corner of the window.

On an Apple silicon Mac

1. Shut down your Mac.
2. Turn on your Mac and continue to press and hold the power button until you see the startup options window.
3. Select your startup disk, then press and hold the Shift key while clicking “Continue in Safe Mode.”
4. Log in to your Mac. You might be asked to log in again.

Depending on your results, you'll want to follow the steps listed under "If the issue continues in safe mode" or "If the issue doesn't continue in safe mode".

If you can't reset your Mac login password, review this resource here: If you can't reset your Mac login password - Apple Support

We hope this information is helpful. Cheers.