I took both the SAT and ACT.
I did terrible on the ACT and since CSU only require to send either one of the test, can I just send my SAT score and pretend like I never took the ACT or will I have to submit the ACT score also?
You can send your SAT or ACT. Since CSU’s superscore it is to your advantage to send all sittings of either the SAT or ACT. CSU’s do not care which test you send, but if you do not plan to send your “bad” ACT, do not report that you have taken the ACT on your CSU application.