Asking for line 91 on form 540 but I don’t have that or know what it is

That is the CA "use" tax and cannot be blank.  If none then enter 0.    The CA state interview asks about it.

What year?  On the 2020 540 line 91 is the use tax

https://www.ftb.ca.gov/forms/2020/2020-540.pdf 

or did you file a 540 2EZ?