I keep trying to engage an enemy and the game blocks me with this message "GDI authority will not let you perform this operation without declaring war".
But the game does not allow me to declare war.

The text about declaring war is misleading. There is no declare war function on Primary. By getting that message in the first place, it sounds like you are attempting to use attack methods other than SS or PS in order to retal, which you cannot do unless they break GDI on you. I'm not sure if your GDI page explains this completely, but it has been asked and explained many many times on the forums.

https://earthempires.com/forum/search

I actually DO have a declare war option that shows up after an initial attack against a target. That being said I have a follow up question. It says i can't use methods that would violate GDI unless they do - how do they do it then if the game won't let anyone unless it's done first by another person - who I'm assuming won't be able to because of the same reasoning. Thank you for taking the time to explain.

GDI is broken by making 2 or more SS or PS on a target. Many players don't break GDI on targets they know can't retal them with SS or PS because they know they will retaliate with harmful non-land gaining attacks and ops. I'm assuming you are looking to use one of those methods because someone grabbed you and you aren't able to SS and PS successfully. This is pretty common in Primary and, again, discussed in several other threads over the years.

Thank you!

If someone attacks me with a missile - will I be able to retal with a gs? Or is that not breaking gdi?