can i file a lien

If you have a valid contract with the current owner of the property, you can and should file a lien, because the buyer will run a title search which reflects the existence of the lien. The buyer will demand that the seller cure the lien before the sale can be completed. You need to meet the applicable timing requirements, as well. See https://www.levelset.com/mechanics-lien/pennsylvania-lien-law-faqs/#:~:text=In%20Pennsylvania%2C%20an%20action%20to,of%20filing%20the%20lien%20claim.&text=On%20all%20projects%2C%20a%20Notice,days%20before%20filing%20a%20lien.