This is a known bug.
Merged your post with existing bug report.
Only thing that works is to become the vet yourself, own the clinic and spay the animal that way.
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There is another way I’ve found to fix this, if you enter the ‘Change Sim’ screen in CAS, you can change your dog/cat to fixed and it works, all without needing cheats!
I also have the same problem in order for my sim to preform the surgery I have to have my sim buy the vet clinic and preform the surgery herself
Well the devs said there's a patch arriving on Tuesday 16th April for America and Wednesday 17th April for Australia and New Zealand. They said it will be their biggest patch for bug fixes since forever according to SimGuruNick on Twitter for those who don't follow. This gives me lots of hope that this and many other bugs that have been such a big nuance for players will finally be resolved. I for one have had to stop playing as my game is essentially broken and filled with game bugs and glitches that have stopped me from playing normally. (And the answer is yes we have all repaired the game many times.) I was starting to reach breaking point as I could no longer play a product I paid money for which is unacceptable. But since the big patch and bug fixes are approaching I think I can finally relax and begin to enjoy my game again.
I searched the notes on that big update they just did, and it didn't say anything about this issue. What's the point of having pets if they're too annoying because we can't spay/neuter them?
I started a new game for testing and - yes!! - spaying my cat in the Brindleton Bay clinic was successful for the first time in months. May be just luck, but perhaps other players should also test if this bug still occurs in fully patched games.
I have the same issue. I can't go to the vet to have a female pet spayed. the vet will go to do the procedure but won't complete it and the machine's screen then looks broken or glitched with static across it with the spay image. you can, however, buy your own surgery station and do the spay at your sims home lot. but that defeats the purpose of the vet sometimes. new save, old, save, no mod, mods... the same result every time.
I tested this several times now in new/old saves with male/female cats/dogs in premade/self-built clinics with good/bad spaying machines and it was successful every time. Looks like the bug is fixed for me after this week's patches.