Gotta target something maybe and you don't meet the requirements. At the top above the list of stratagem it says stratagem and target. I know some thing s are not allowed for certain stratagems.
For a number of strategems you need to select a target in the second tab. Even then it might be impossible because of different reasons.
For example.. the strategem "research cooperation" you need to select a valid major power in the second tab(2) (which means you need recon on that major(2)) and even then they need to have the right goverment type(3) to be allowed... so thats three possible reasons this strategem can fail so to speak...