MacOS Mail - "Important" Folder is Grayed Out - Inaccessible

Gmail?

If so, go to Gmail web portal Settings > All Settings > Labels and turn off "show in IMAP" for all of the odd folders.

Gmail tags messages. It does not file them in a folder. It can tag a message with many "Labels" thus making the message appear in many folders.

The most problematic is "All Mail" which Apple Maps into the Archive folder. Since every message is tagged with both an "All Mail" label and an "Inbox" label, messages will land in one or the other through random selection.

Better yet, don't use an email client with Gmail. Just use the web portal as it was designed to be used.

When I turned off "show in IMAP," all the gmail emails disappeared from MacOS Mail. I turned show in IMAP back on, and they reappeared. I set every folder in Gmail to show, as well.

Just disable the goofy, superfluous ones like Important, All Mail. Any you turn off won't show up in Mail because you told it not to.

You may have to remove the Gmail account, then re-add it to Mail after adjusting those settings.

You may have to also go into the web portal to re-add the Inbox label to the Important messages as it might have been stripped. Same for All Mail.

I actually want to use MacOS Mail, as I can view all my email in one spot without opening multiple browser windows.

Then you will have to continue to fight the non-standard IMAP implementation that Gmail pretends to use. Maybe try a different email client that supports Gmail goofiness.