PRP causing scar tissue - will it make it hard to get a hair transplant if needed?

PRP administered by an expert should not cause scar tissue. In fact, PRP is used to treat scarring hair loss. The one exception may be if a person is a active Keloid former. However, it's very rare to see Keloids in the scalp. In summary, Don't be concerned about PRP leading to scarring.
PRP treatment is effective for preventing hair loss and assisting regrowth while undergoing treatment and for several months. Find a PRP expert.
Jeff Rapaport MD FAAD
Realself Hall of Fame

The likelihood of scarring with PRP if injected with good technique and prepared in sterile condition by an expert injector is very low.

PRP done properly, by a skilled provider into the correct space in the skin, should not result in scar tissue. PRP can prevent hair loss, and even reverse it in some cases, so long as you maintain the PRP. PRP prior to transplant surgery can optimize the recipient area as well as reduce the area needed to transplant, both in terms of size and density. Those who do PRP may also be less likely to experience shock loss with their surgery.

Knowledge of PRP is growing.There is no evidence to suggest that micro-trauma from micro-needling or PRP injections would make a hair transplant difficult.Scars in the donor area or scars in recipient areas can make a hair transplant a little more challenging, but they are not insurmountable challenges.In the long run, anything that is proven to help prevent further hair loss, is a welcome addition to the hair restoration journey.