I purchased a trial subscription from Firefly, but I can never create any content; it constantly gives errors. It's an absolutely useless application.

Hwy ​@Çanakta 

Depending on the application you may not have enough credits?
Each video application and model costs/ uses a certain amount of credits.
 

 Generative Credits Different Plans | Options

https://helpx.adobe.com/creative-cloud/apps/generative-ai/generative-credits-faq.html#standard-and-premium-generative-ai-features

Each Generation will indicate how many credits is required

Cheers

Nate