Adding secondary email address to my Apple Account

I used to own an IPad mini back in the day, which had an Apple ID that was connected to my private email address. However, recently I got a new MacBook Air for my job, for which I had to use my work email address to make a new Apple Account. As a long time Windows user, I am excited to try out MacOS and see whether or not I like it, so I suspect I will use my MacBook for personal use as well.

Therefore, I wanted to set my private email address as a secondary in my Apple Account. I then found out that my email address was still tied to another Apple Account, deleted it a few weeks ago (since I don't ever use an IPad anymore anyway), and tried to set my private email address as a secondary today, but it still says that I cannot do that. (I have checked that I cannot login through my private email address anymore)

How do I add my own email address as a secondary email address to my Apple Account? I have an outlook email, is that the problem?