Unable to submit self assessment tax return through commercial software suppliers

I'm trying to submit my tax return through commercial software suppliers, but can't submit due to said " credential errors" which need activate the self assessment service on HMRC. Unfortunately when I login my HMRC account, there is NO window for me to do the activation. I have followed the software suppliers information to do, it's No window could find on HMRC web. Can you team teach me how to activate it? Really frustrated.

Hi,

It sounds like you do not have a live Self Assessment record. You would need to register.

Self Assessment tax returns

Thank you.

Hi, thanks for the reply. No, I did registered the Self Assessment with the UTR no. assigned to me, I also received the official letter from HMRC about that.