I'm buying a home a few weeks before closing I found out there is a lein. Should I as a buyer pay the lein or back out?

Under 99% of the contracts I see, the seller is obligated to give you clear title, which means no liens. The seller should pay off the lien; often it is deducted from the seller's proceeds at closing. If the lien exceeds the sales price, the parties can negotiate with the lienholder to see if  they will take less.