You are wrong.
You can send bitcoin from any type of address to any type of address you want. This means that you can send bitcoin from segwit address to legacy address and vice versa.
Well that's Strange, cos I know I tried it sending Bitcoin from a Legacy address to a SegWit address, and I'm pretty sure it didn't go through.
Well, it's even more strange that you don't provide any details from which wallet you tried your transfer where it "didn't go through" as you remember. It's also open for interpretation what you mean by "didn't go through". Could be from showing an error to too low or otherwise unsuitable transaction fees that prevented the transaction to be finalised.
While I know that @hosseinimr93 is right, I can only assume that you tried your transfer from some braindead (online?) wallet that doesn't support withdrawal or sending to native segwit addresses. There may still be services out there that don't support native segwit addresses and I'd rather choose to avoid such underdeveloped things.
I believe the wallet I used was called the Mycelium Wallet, that was the first time I tried using the wallet and I can't tell the reason the transaction as wasn't successful