does the email have an invoice number and case number?

I had the same issue. I received a welcome letter on Friday, but couldn't log into my account. I sent an inquiry on CEAC official page and was it a coincidence or it worked itself all of a sudden after 5 days, I can't say. They give 72 working hours for activating the account. I guess you need to wait a couple of more days

So I received the letter on Friday and it gave me access on Tuesday