Kimasu(come)- KORAREMASU(can come in Teinei Form)- Korareru(can come in Futsuu Form)
can't come in Teinei Form= KORARE-MASEN
can't come in Futsuu Form= KORARE-NAI
You might hear "KORE-RU" as for potential verb in the conversation very often.
However this is grammatically wrong, called "RA nuki kotoba" (omitting "RA", RERU instead of RARERU)
Hope it's helpful explanation.