"It's not you, it's us" when accepting IAM Identity Center invitation

Hello.

How can I recover from that? Should I delete the user and create again?

Yes, I think it would be easier to recreate it.

Is MFA mandatory for administrative users created that way? When the invitation was showing me the MFA options, it was stated that my AWS organization requires MFA. But it was me, who created that AWS organization, so I should be probably able to turn it off. The tutorial implies it's mandatory though.

I believe that IAM Identity center is set by default to require MFA to be registered at sign-in.
https://docs.aws.amazon.com/singlesignon/latest/userguide/how-to-configure-mfa-device-enforcement.html

If MFA is forced in IAM Identity center, I think the settings in the following document are related.
Please sign in as an administrator and check the settings in the document below.
https://docs.aws.amazon.com/singlesignon/latest/userguide/mfa-getting-started.html

It may be possible to resolve the issue by setting it to "Allow them to sign in".

After clicking the "Accept invitation" link once in the invite email, I think the link is no longer good. The "It's not you, it's us" error message should probably say "Invite was previously accepted already" or something similar.

In the email, it's easy to miss this text below the invite button link:

Accessing the AWS access portal

After you've accepted the invitation, you can sign in to the AWS access portal by using the information below.

So for future logins, use the "Your AWS access portal URL" link from the email.

Once you're logged in, instead of the dashboard (which I was expecting to see) you'll see a list of Accounts you have accepted invites to. Select the Account you want to manage, then select which Role within the account you want to use (each Account can have multiple Roles with varying permissions is my understanding).