They could if the tax lien was after filed the background check was conducted.
But to maintain the security clearance, you are supposed to report adverse information like going into debt, having missed payments, etc.
Having a lien filed after the last review process and not reporting it is in and of itself a security violation. You’re supposed to report ALL adverse information - including debt like this. And unpaid tax debt is pretty obvious, whereas personal loans run up are harder to find.