Error?

Same here.  I heard reports of it not working last week but it was fine when I checked it.  Now it's pretty well broken, except for booking as a guest.

Tried Edge, both regular and inprivate.

Tried Chrome, both regular and incognito.

One report came from a mac user, unsure if they were using Chrome or Safari.

In all cases I tried, when logged into a Microsoft account, I get the error in the original post.  Tried multiple booking pages and booking from multiple accounts (although all in the same tenant).

In all cases, if I "Continue as a guest" and provide a non-MS email, it sends a verification code to that email and then successfully creates a booking.