I can make calls to non iPhone users but I can’t send text messages, why?

Calling and texting are two different things. While SMS uses a portion of the cellular voice channel, you still need the carrier to ensure provisioning on your account for SMS/MMS.

Text messages to non-iOS users is an SMS. An SMS is a carrier feature and there are no settings on the iPhone to set that. It needs to be configured on your account by your cellular carrier. Contact them for assistance in troubleshooting this issue. Also, make sure your iPhone is updated to the latest iOS, 14.2.

What previous post said. SMS or text messages are wireless carrier feature, you have to contact them with issues with text messages.