It's either one of two reasons she told you that:

1 she has other plans

2 she's lying because rejection is tough to do

So now we've already asked her out and she's given a "soft rejection". Best case scenario, she offers another date and/or time. If she doesn't offer that you don't want to be needy and keep asking, (you've got better things to do with your time!). So what do you do?

I've used two approaches, both with equal success.

1 Ask "When are you free?" immediately. If she responds positively to this then you're in business. Set up a date to meet up.

2 Tell her "I already asked you out once, it's your turn this time". Kinda turns the table on them a bit in a fun flirty way.

If she responds poorly to these messages or ghosts you, she was never planning on meeting up regardless of what you do. Delete her and move on.

Hope this helps someone.