Even with a paid plan, you can still need credits for generation. How credits are consumed depends on which plan you have and whether you are generating using standard or premium features. Your image does not completely show what you were doing (it is best to upload an image of the full browser window and not just the error message), but it seems like you were using a Google model. Google models require a plan with premium features and when you generate they consume credits as documented in the partner credit page.