That usually means you already have another companion quest active.

Since they require the companion to be locked into the party for the whole quest, you can only have one active at a time. If another companion tries to initiate their quest (when you speak with them), you just get the "not now" dialogue.

If this is the case, you'll need to finish whatever quest you have active, and then you should be able to speak to Niesha and start her quest (there'll be an option to ask what she wanted to talk about).

know this is necro but TY
was driving me nuts