Check that your carrier plan includes SMS/MMS messaging
Turn off Airplane Mode
Make sure Cellular Data is turned on
Restart your iPhone
Go to Settings > Messages and turn on iMessage
Go to Settings > Messages and turn on Send as SMS
Go to Settings > Messages and turn on MMS Messaging
Go to Settings > Messages and turn off Filter Unknown Senders if needed
Check if the contact is blocked in Settings > Messages > Blocked Contacts
Remove the contact from the blocked list if needed
Update your iPhone to the latest iOS version
Reset network settings in Settings > General > Transfer or Reset iPhone > Reset > Reset Network Settings
Contact your carrier to remove message blocking from your account
Ask your carrier to verify short code, international, or premium messaging restrictions
Check whether your phone number has been suspended or restricted by the carrier
Reinsert the SIM card if you use a physical SIM
Contact Apple Support if the issue continues