This appears to be an error that is still being fixed. I've tested add column in my own test environment and it doesn't work either. But you could achieve it by adding a column from Teams SharePoint site.
- Go to Files tab < click on "Open in SharePoint".
- Click on Setting button (On right top corner) < Library settings < More Library settings < Create column.
- Fill in name, category and other information for the column < OK.
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.