The following error displayed below is a result of a bug in the Windows Admin Center. I was able to successfully create a scheduled task on a monthly basis in my lab environment as shown below https://techdirectarchive.com/2020/03/26/how-to-create-a-scheduled-task-with-windows-admin-center/
I found this error was reported also in Windows Server user-voice
– Note: Daily tasks can be created, edited, run and deleted successfully.
This issue comes up when you select the trigger on a monthly (monthlyDOW) basis. I am sure this will be fixed soon.