When I use the objective ability, the attacking enemy unit get a focus token on them?

Thanks

Only if the unit can generate 2 resources and its the only way for your opponent to meet the requirements of spending 2 resources (i.e. All his other resource generating cards are focused), and he would get two focus tokens. If it was a one resource unit and your opponent could generate a resource by another card it would get one focus token. If there is no way for your opponent to generate two focus the engagement ends. The Reaction is generating resources not focusing units.

I guess what I meant is what happens to the attacking units? Are they focused because they attacked? Are the ready to dacrlare as attackers on another objective?

Thanks again!

Once he declares attackers and can't spend 2 resources the engagement ends without further resolution. So there are no defenders declared, no Edge battle and no units are focused to strike.

The one question I have is does the action window after declaring attackers happen or does the reaction prevent it.

The one question I have is does the action window after declaring attackers happen or does the reaction prevent it.

It's a Reaction to attackers being declared, which means it happens immediately after attackers are declared and before the Action Window. If the Light Side player does not pay 2 resources, then the engagement ends immediately, which includes not advancing to the Action Window between declaring attackers and declaring defenders. There is still an Action Window available between engagements though if you wish to use an Action before the next engagement.