Hi Raghib, thank you for your attention.
Unfortunately it still didn't work. What I did was a big mess and I think this is causing the problem so far, even preventing me from creating an automatic Sketchfab account and associating with the Epic Games account.
I haven't mentioned this before, but I managed to use Sketchfab once inside Twinmotion (before all this mess), which created the account with the nickname "Drelius" (which was the same username I used at Epic, and used an [at]yahoo.com email). However, days later, when I saw the email saying that this account had been created, I remembered that I had a previous Sketchfab account (which I never associated with Epic).
Don't ask why, but I actually had TWO older sketchfab accounts (andreviz & andre.viz, using respectively rocketmail/gmail emails as logins). And then, the "Drelius" one, created by Twinmotion and deleted shortly after creation. Yes, I know I made a big mess.
So I decided to delete the new Sketchfab account (created by Twinmotion), and because I thought that if I used the same email address in Sketchfab and Epic, it would allow direct recognition by Twinmotion, I recklessly changed my email from Epic login to the same email I used on my older Sketchfab account (which I know is a problem as I can only update it again in 3 months). Since deleting the "new" Sketchfab account said that the previous username could not be used, I also updated my nickname on Epic, to test it out (another restriction for 2 weeks).
I noticed that it didn't work when it always gave an error message when trying to link the accounts (I tested the association with the 2 old accounts). I got a message saying that the email was already being used by another account... What I can only conclude is that the fact of using the same email on Epic and Sketchfab causes some conflict for the automatic creation or association.
After this topic, I contacted support, but the instructions were similar and it didn't work. In the end I don't care about this issue of old accounts on Sketchfab anymore, as I didn't use them that much before, I just wanted to know if there is any other way to solve this now, without having to "try" to wait to change my email from Epic 3 months from now.
Another thing I noted, as you said the Sketchfab team has deleted the account from their database, I think it was the "andreviz" they deleted, because I tried to login or find this account, and it is not there anymore. It was the one associated with the "rocketmail" email (which I guess causes the problem to login with Epic account, as I changed the Epic email to this rocketmail one). But it was not the one which Twinmotion created (Drelius, with yahoo email)
I really apologize for so much chaos! 😫 😥