I've tried everything and I can't receive or send SMS messages. Ideas on what to do next?

If you cant sent or receive texts ... and calls , that means sim is not provisioned . But if you can receive calls or make them but no texts , call your carrier or simply follow this link : If you can't send or receive messages on your iPhone or iPad - Apple Support

Did "everything" include contacting your carrier? SMS messaging is a carrier function.

SMS Messages (The Green Bubbles) require no setup.

It works if you are connected to your carrier and your plan is open for sending SMS messages, and you have cellular coverage where you are. This requires that you have an active SIM card (or eSIM) in your iPhone, and it requires that your SIM card is connected to your Carrier, this typically happens after you have entered the SIM card's PIN code. If your SIM card does not use a PIN code, the SIM card is automatically connected to your Carrier when your iPhone is switched on.

If you use eSIM, check if the SIM card is active here:

Settings → Cellular → eSIM

Or should you, as IdrisSeabright suggests, contact your Carrier.

If you can't send iMessages (The Blue Bubbles), tell us, or see this support article:

If you can't send or receive messages on your iPhone or iPad - Apple Support