Mail cannot save informat about your mailboxes because there isn’t enough space in your home folder.
My mailbox cannot save information about your mailboxes because there isn’t enough space in your home folder.
MacBook Pro 13″, macOS 11.6
How full is your disk?
Launch Pad > Disk Utility and see how much open space you have on the drive.
You need 15%-20% of the drive totally unused at all times and if you have a small disk this can be a challenge as where to store your user data. If the drive is "full", you need to move user data to external storage, but not iCloud.
Once you're freed up space, if that's the problem, you should be able to proceed.