Hello from Slovenia, After a little less then a month of going back and forth with apple support, trying different things and struggling with issues regarding buying the account, today i finally got it resolved. Here is the story:
At first i enrolled (tried to) through the web, I didn't even make it to the payment, I tried two different apple IDs, same thing. Then I reached out to support and they've said to enroll through the app. Again, couldn't even provide my ID images (unknown error occured) again, I reached out to support and they sent me a link to upload the IDs through the web.
They manually authorized my ID and I was able to get to the payment stage. When I tried to pay, again: "Your purchase could not be completed, purchase failed an unknown error ocurred, etc. Again I reached out. Turns out the support made a typo in my surname, but still couldn't pay. Again I contacted support and they called me. Then they transfered me to Itunes support. I called them and we were on the call for 40min. They couldn't resolve it so they said the issue will be passed to higher developers or something and I had to wait for 24hrs and try again. I've waited and tried again and it worked.
So to sum it up:
- Stick to one apple ID (probably the one You use most)
- Use the same device for the whole enrolment process
- use the apple developer app
- keep contacting apple support as the problem is probably on their end
- I also used Revolut for the payment
- DONT GIVE UP, try everything, reset your iphone, remove all payment methods, add them back, try different ones, update the iphone and all apps...
Hope this helps someone