If you have a negative Paypal balance and don't pay the Negative Paypal balance then eventually within 3 to 4 months and sometimes sooner your Paypal account will be closed and if the negative balance is large enough then Paypal might send it to collections and it could ding your credit report and score.
If it's a small amount then contact Paypal and work out a way to pay it off.
Having a Negative Paypal balance can affect you getting a new Paypal account or ever using Paypal again.