https://support.office.com/en-us/article/Office-for-Mac-repeatedly-requesting-keychain-access-ced5a09c-3099-47cb-9190-e961bf63e240

I would do the deletion of keychain and resignin to office. If that doesn't work, backup your computer and then hit always allow. Make sure your backup isn't connected to your computer just in case this is something scary. I wouldn't be too scared (just being overly cautions) -- it seems like a common case.