Hi, thanks for posting. The “This Month” tag doesn’t include a bulk delete option because Outlook only groups daily and weekly emails for that feature.
To remove all emails from that month, select one message, hold Shift, click the last message in the list, then delete. You can also type “received:this month” in the search bar, select all, and delete.
Microsoft doesn’t currently support one-click monthly deletion, but you can send feedback through Help > Feedback in Outlook to suggest it.