"Mail cannot save information about your mailboxes because there isn't enough space in your home folder. Quit Mail..."

[usonian]

1. I literally typed your title into a web search.

1. There are simple things to do, like deleting old messages, attachments, and emptying trash.

https://www.devicemag.com/mail-cannot-save-information-about-your-mailboxes/

Try it. This is always good to do, and then:

2. There are complex things that can be going on.

https://discussions.apple.com/thread/250717871?sortBy=rank

In that case, call Apple support, or hit the genius bar if one is near you.

DON'T FORGET
Apple will let you (by default) store your home folder in iCloud. I'm a techie and I want my home folder on the disk in my mac.
If you have the former (It's under your Settings/iCloud/Optimize_Mac_Storage) you might be over your free quota (or your paid iCloud storage) But if you got to step 2, let Apple help.

Having the computer "hands on" is a quadrillion times easier to deal with than people trying to guess what's going on.
Because it often means checking a bunch of obscure settings.

💻 GOOD LUCK! 💻