If you take maca root, can you still get a BBL?

I am unaware of any medical studies reporting that Maca enlarges the buttocks.  They may help you gain weight, but I rarely recommend gaining weight prior to a BBL. Gaining weight is often counterproductive. If you gain weight that is fat, some of the fat will likely be associated with the buttocks. 
Recommend that you schedule a virtual (it's typically free) or in-office consultation (fee usually applies to surgery fee) with a board certified plastic surgeon in your geographical location who can examine your anatomy and recommend the procedures which would provide the ultimate goals you are seeking for your buttocks.

It is advisable to not take any herbal or other supplement a week before a BBL and one week after. See a board-certified plastic surgeon who does a ton of Brazilian butt lifts and operates at a certified facility, and you will be in good hands. Ask that surgeon; how many Brazilian butt lift did you do last year doctor? Do not settle for vegue answers.

The maca root, from what I have read, is a plant native to Peru. It is dried into powder and is available in powdered form or in capsules or as liquid extract. Its purported effects include increased libido, increased fertility,  and mood / memory elevator. 
The drug is known to interfere with normal thyroid function and for this reason, thyroid function studies are recommended before any elective surgical procedure. Otherwise there does not appear to be any contraindication to performance of BBL.
Seek a consultation with a board certified plastic surgeon.
Best wishes...