Hi @Candy ,
Welcome to our forum!
As I know, this is a recent known issue: Can't create new signatures in new Outlook for Mac and existing signatures are missing.
Aggree with the workaround in the thread as AndreasBaumgarten provided, it's suggested that you could try to switch to Legacy Outlook.
In addtion, if the known issue has any update, i will post back here as soon as possible.
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.