Unblock premium text messaging to short codes

Hi I cannot send text messages to short codes on my xfinity mobile phone.  I've tried everything to fix but the options do not exist on my S21 Ultra it seems.  If I go to Enable Premium Text Messages, it just says no apps but does not provide an option to allow Messages.  If I install an alternate SMS program it makes no difference.  When I send to a short code, first I get a warning and there is a check box to acknowledge but then no way to tell the system that I want to proceed and then I get a message not sent notification.  This is really annoying!!!