hi,
I will give you my scenario so it is clear how i fixed it for me:
When I got the classic free teams, I had not bought an Office 365 yearly subscription. I has still using my old Office installed via CD in an older computer, cause I had a work laptop with newer office.
So I got MS Teams and I recall it sounded coll to have a tenant as well and play with Azure.
The tenant name I gave at that time was in the following is below:

That primary domain is where my very original Username, that I created back in 2019 was atttached to.
So my original user i will call it for this purposes "olduser"
so the email i used to log into entra ID, was "olduser" AT storaenso211DOTonmicrsoft.com
That "olduser" is what was showing in error in my MS TEAMs with the triangle error icon, underneath my newest MS Teams application

So logged in as "olduser"
So what I did was to create a new user in entra, I will call it "newuser"

I gave all Admin permissions to that "NewUser", and you create a password to log into Entra

Then I logged out of entra as "olduser"
I logged into Entra as "newuser"
and since "newuser" is also a PowerAdmin, i could delete "olduser"
Not having "olduser" in Entra, fixed my proble in my new MS teams.
Now, if you still want to delete your tennant because you are no longer going to play with it, that you need to request to Microsoft Entra team.
But before they can delete it for you, you need to ensure there are no subscriptions, and that is step 2
After you have done the above, do this.
See how your Microst 365 Office Admin looks?
So go here:

it must likely shows that you still have an "Enabled" subcription, but it is Unassigned"
I do not have a screen shot of how it looked when it was enabled, but i did keep one, of when i selected it, and chose to Disable
so it looks like this after you do that.

For some reason for me, it did not let me Delete it, so i had to wait until it as automatically deleted after the date that showed up (in my case it was automatically deleted on 16th August)
not when i log into Microsft Admin Center it loosk like this.

and thus I am able to request to have my tennant deleted if I want because my tennat status looks like this

Hope this helps!
Cheers!