Does not seal - Rival Crock Pots & Slow Cookers

Your seal has gone bad. New seal.

https://www.ebay.com/p/Wolfgang-Puck-Replacement-Sealing-Gasket-for-8-Quart/23019672466 https://www.ebay.com/p/Wolfgang-Puck-Replacement-Sealing-Gasket-for-5-Quart/17019666266

cook ware shops
seal and "O" ring retailers
e-bay
manufacturer spares

as the cookers are a rather simple unit , the manufacturer seldom keep spares
however the seal is most likely a generic item so take it as a sample to a homewares shop to see if there is a seal available
if it is a "O" ring type then there are retailers that specialize in seals and "O" rings and one could be made to suit the unit

Go to Goodwill or thrift store and buy a cheap lid and replace your seal with the one from that lid.

go google and type in -- user manual for mirro pressure cooker model m-0536-1-- and if there is one it will be a pdf version down load
make sure that the lid seal went in without any twists
put something under the seal and go around the lid several time to allow the seal to untwist

Make sure rubber in lid is in place correctly. Also check the lock.