Difficult to see from your screenshot only. Try to upload your part using advanced editor.
Anyway it seems you are trying to cut the fillet like surface using the green surface. The trim tool must intersect the whole surface-BODY to trim. Trim function trims entire bodies only. And your green surface only intersects one of the fillets.
If you want to trim only one surface belonging to a larger surface body, use Split-Line and if needed Delete Face afterwards.
It looks like the surface you are trying to trim is knit to the surrounding features which would mean your trim surface does not completely bisect the target surface. either extend it out in all directions or create split lines with the surface and delete the unwanted faces
Thanks guys, I'm still very new to Solidworks, didn't even think of using the split line option since I've never used it before.