The log-in page is https://www.gov.uk/personal-tax-account. Perhaps you haven't actually set it up yet? If you are in the old-style Self Assessment area (styled with green menus), you could try clicking on 'Your HMRC services' in the top-left and you should be presented with a link for your Personal Tax Account.
Yep, It is the green one. . . I did find your mentioned "hmrc services" link (thanks). . . it showed a page (been there before) which does list "personal tax account" link (seems the perfect place to go) . . .
But. . . clicking just asked security questions. . . then opened up with another (same) "green" (business) page. . . same as where I'd started.
POSSIBLE route to solve:
I tried the Personal Tax Account link you gave. . . it goes to a page "Provide your identity" with:
3 signin options plus 1 "create an account"
Now this. . .
I've been here before and, because I have a Gateway number, I always choose the first signin choice. . . "Sign in with Government Gateway". . . but it just goes to the green SA area.
So. . .
Should I here. . .
Click the "create account". . . and be creating a new account. . . A Personal Tax Account?
thanks