How to fix "Selected user account does not exist in tenant 'Microsoft Services' and cannot access the application '18fbca16-2224-45f6-85b0-f7bf2b39b3f3' in that tenant. The account needs to be added as an external user in the tenant first. Please use a d"

Question

Hello. I am trying to learn for the az-900 certificate using Microsoft Learn platform. However, I am having some real problems with the Sandbox. When I try to access it, the login screen pops up and after I select my Learn account it shows me that message: "Selected user account does not exist in tenant 'Microsoft Services' and cannot access the application '18fbca16-2224-45f6-85b0-f7bf2b39b3f3' in that tenant. The account needs to be added as an external user in the tenant first. Please use a different account.". I have no idea how to resolve this issue, I tried searching the web with no success or even understanding what is wrong here. Don't tell me about incognito mode or different browser please, I've already tried all of that and the result is always the same.

And sometimes another message pops up: "AADSTS500200: User account '/my email/' is a personal Microsoft account. Personal Microsoft accounts are not supported for this application unless explicitly invited to an organization. Try signing out and signing back in with an organizational account." But what is the point then? I think that I must use Learn with my personal account (as it suggests during the registration).