I had to get a fitnote for universal credit for my autism but it only lasts 2 weeks?

I was told I could benefit from getting a work capability assessment for my autism so I went to get a fit note from my doctor

She gave me the fit note but it will have to be renewed after two weeks, and when I submitted the fit note I was told I wouldn't be able to get an immediate referral for assessment because my medical evidence (the fit note) doesn't last more than 29 days

I was also told the first fit note couldn't last more than twelve weeks

I don't understand why my first fit note only lasts two weeks, especially because this is a continuous condition, I'm not gonna be cured of autism in two weeks

Am I going to have to get this renewed multiple times? Will I have to get a new fit note that lasts more than 29 days?