try removing the rubber seal from the valve cover/engine head,
give the rubber seal, grooves in the valve cover that the seal sits in and the head surface that the rubber seals against a good degreasing n wipe dry with brake cleaner/carb cleaner/isopropal alcohol on a cloth
lay a bead of PU gasket sealant along the groove of the valve cover
press the rubber seal into the valve cover groove (clean excess sealant if needed)
lay a bead of sealant along the top of the engine head
carefully lower the valve cover & seal in place in one go
tighten the valve cover screws
wait several hours for sealant to fully cure before starting engine