On Thursday, sir? The time is very short.
My father Capulet will have it so,
And I am nothing slow to slack his haste.
You say you do not know the lady's mind.
Uneven is the course; I like it not.
There's your answer, right at the start of the scene. Friar Laurence is worried that the marriage is happening too soon ("the time is very short"), and secondarily, that Paris has not had Juliet's consent for the wedding yet (he does "not know the lady's mind").
Of course, there is another level here. Friar Laurence knows fine well that Juliet is already married to Romeo - he actually performed the service. He has no idea what will happen if Juliet's wedding to Paris is properly on the cards: nor does he know what to do.
Juliet cannot, legally, and will not want to marry Paris. She is already married. Can he secretly divorce her and Romeo? - no, she'll (and he'll!) never allow it. So there is a real problem to be solved. And it's later in the same scene that he comes up with the solution.