This happened to me last season trying to buy a 1/2 season ticket. Phoned up to buy my ticket and get them to fix this problem for me. still get that message when i try to log in with my email address but i can use my rangers number.
Getting my first full season ticket, i got a half season ticket last season when we went into admin and kept my seat this year. I work most Saturdays so that's always stopped me in the past but going into admin was a wake up call.