I see two possible solutions from now on:
1. As the poster above stated, say you were only unsure because of your shyness. Especially because she's a bit older and looks more mature. Which shouldn't be bad, but you just didn't know what to do or say out of amazement. Or simply you're not used to dealing with these things and you felt too much pressure to do the correct thing which is almost the same as the first reason. Or say just one of these things or a variant you think of yourself and get it right. Everyone does clumsy things when they're that young. This must be one of your first contacts of that nature with females. That's supposed to be NORMAL I guess. Just saying you're a shy person and you feel unsure should never hurt here. Or that you didn't know what to think of the situation because you barely know her and she didn't ask this in person, but sent a friend. This proves almost certainly that she's not that confident herself. Or she realizes well enough it's odd to ask out someone she doesn't know that well.
2. Tell her you now know for certain you'll be able to spend time with her. There were some situations you had to take care of first. Or an appointment or other arrangement to move to another moment/do. Make something up. Every teen has some situations, it may be at home or elsewhere. Like asking your parents whether it's okay you'll be away for that time.
Second round, go out and tell her NOW!!!
Then again, girls are strange creatures.
I don't know why I add this, but just keep this in mind when things don't go as planned, even if you don't foul up this time. :spotted owl: