If I forgot to sign my CA state return and they send it back after April 15 will I get a late fee?

you might, but I doubt it. Be sure to pay on time if you owe.

You might. California states:

If you file your tax return or pay your tax after the due date, you may owe interest and penalties on the tax due.

If you have other questions about this, ask in the comment section below.

California Instructions:

https://www.ftb.ca.gov/forms/2018/18-540-booklet.shtml#18_540-5