Many areas , I know NY is one, auto parts stores are no longer allowed to sell threaded plugs that will fit brake line fittings. Brake fittings are straight thread where NPT is tapered , so they won't fit.
When I need to plug a line as a temp. fix , I cut the line flush with the fitting and use my wire feed welder to plug the hole.Then reinstall the fitting.
Not a good idea but it lets me get to the parts store and back so I can fix it the right way.