I have just Migrated using migration assistant for a standard user to a new MacBook Air.

When you initially set up the Mac, did you elect to transfer information from the PC according to Move your data from a Windows PC to a Mac? If so that was absolutely the right thing to do, but the migration should have resulted in a User account with Administrator privileges.

If you are unsuccessful invoking the "new user setup" described in that Discussion, you might be better off erasing the Mac and starting the migration again: Erase all content and settings on Mac - Apple Support.

Erasing everything probably requires Admin authentication. Since there are no Admin accounts on that Mac you will need to scroll down to the passage under If you can't use Erase All Content and Settings.

At some point the Mac should have asked if you want that User to be able to administer that Mac, which you obviously want to do. The fact there appears to be no Admin accounts on that Mac isn't supposed to happen. That alone is justification to contact Apple and have them walk you through it: Get Support.

Please let me know what you find out either way.

I was afraid of that. Please refer to this Discussion: administrator changed to standard: cannot log in without details: single user mode fix does not work for me...

That link will take you directly to the proposed solution, so try it. Just beware it was written for an older macOS version and an older Mac, and it may not work for newer ones. You won't hurt anything by trying it though, just pay attention to the caveats.

If you have questions or can't use that procedure for whatever reason, please write back and tell me where you got stuck.

You need to log out of the Standard account, and log in as an Administrator.

Great! Thanks for the update.

It shouldn't have been so difficult but I'm glad it worked out.