Should I amend my 2020 return to include a 1099-G form I had forgotten? If so, how?

Was it less than 10,200? If so, it may not make a difference on your return. The first 10,200 of unemployment for an individual is not taxed for 2020. This was a recent tax change and the IRS is temporarily blocking amended returns that claim unemployment so they can try to get caught up. 

Did reporting the 1099-G cause you to owe additional tax? If so, make the tax payment. 

Print the 1040X and mail it in. 

It was less than 10,200

You can't amend before the original processes. 

In your situation, you won't owe additional tax, so first wait and see if the IRS corrects everything for you. 

Do you also file a state tax return? If so, which state? 

I filed a state tax return and already been deposited the refund. I filed for California

CA does not tax unemployment benefits.  You do not need to amend your state tax returns.

I filed my 2020 taxes in January an had no idea about the 1099g form an when I finally got the form in March I tried to amend my taxes but it wouldn’t let me do I still need to amend my 2020 taxes cause I forgot my 1099g form for unemployment?