Hi, the exact same thing is happening to me (I'm on PC, and I don't have any CC, or Mods in my game). I thought maybe the issue was occurring because my sim's a mer-person, and maybe was trying to turn into their mer-form, because they were trying to wade out to where the fish sfx was (I know that's a reach, but I was trying to come up with a reason why it was happening, even though I know for a fact that sims don't wade out towards the fish sfx when fishing in Sulani), and then I checked with a non-merfolk sim, and they still couldn't fish either.
My theory is it might have something to do with the fishing signs being on beaches, and going into the ocean/sea (I'm pretty sure Sulani is the only world that has beach fishing places; I couldn't find any in Tartosa, or Tomarang, but I might've missed them, and I can't think of any other world that has a beach-type fishing spot).
Hopefully, how I worded everything makes sense😅.