Fix MM64178 (#30957)

Co-authored-by: Mattermost Build <build@mattermost.com>

server/channels/app/team.go

@@ -490,6 +490,10 @@ func (a *App) UpdateTeamMemberSchemeRoles(c request.CTX, teamID string, userID s
 		return nil, model.NewAppError("UpdateTeamMemberSchemeRoles", "api.team.update_team_member_roles.user_and_guest.app_error", nil, "", http.StatusBadRequest)
 	}
 
+	if !isSchemeUser {
+		return nil, model.NewAppError("UpdateTeamMemberSchemeRoles", "api.team.update_team_member_roles.unset_user_scheme.app_error", nil, "", http.StatusBadRequest)
+	}
+
 	member.SchemeAdmin = isSchemeAdmin
 	member.SchemeUser = isSchemeUser
 	member.SchemeGuest = isSchemeGuest