I don't know, exactly. It never accepts numerals for me, either. I have reported it several times, but nothing seems to have been done about it.
It's a set phrase that you just have to accept :)
"To have to do something" = "Tener que hacer algo".
"Tener que" has a colloquial Spanish meaning that can translate to "I must."
So the other responses are somewhat correct, but it's better to understand the broader context. When you link verbs (such as here, using both the verb "to have" & the verb "to leave") then you sometimes need to add a preposition between the two verbs. The required preposition depends on the first verb.
In this case, when you link an additional verb to tener, then you need to link the second verb using 'que'.
However other verbs require different prepositions. You might have come across constructions with 'trying to do something', for instance "I try to eat". The preposition depends on tratar (to try) which takes the preposition 'de'. I try to eat is trandlated as (yo) trato de comer.
For more information on linking verbs, and lists of the associated prepositions, see http://users.ipfw.edu/jehle/courses/VRBSPREP.HTM