I've never experienced any problem creating a series on Tapas, unless they've changed their method.
Back when I registered it was:
- Create an account
- Go to dashboard
- Click "New Series", select if comic or novel
- Then fill on the info, chapter, load files and such.
Never had to go to creators.tapas.io, which by checking myself, no I don't have access either.
I simply go to tapas.io/dashboard/index then create my series.
What seems to be the thing is that said link you got is for Original creators that have a contract with Tapas or must be an outdated feature.