There are two main reasons why you may not be able to switch between GTA 5 characters. The first one is that you could be on a mission that doesn’t allow character switches. The other reason may be that you’re too close to a mission. If you’re too close to a mission point, the menu that typically brings up the character screen may display the radio station screen instead. Drive about 10 seconds away from the mission point and try again. If neither of the two reasons happens to be the case, contact the Rockstar tech support to help you solve this issue.