Issue in cypher query using if if and merge : In given cypher query it showing error.Please let me know what,s wrong in below cypher query

MATCH (pcp:PCP)-[]->(mt:MONEYTRANSFER{MTCN:'1618087665684829'}) return CASE WHEN  EXISTS( (pcp)-[:SENT_BY]-(mt{MTCN:'1618087665684829'}) ) THEN null ELSE MERGE (pcp:PCP)-[]->(mt:MONEYTRANSFER{MTCN:'1618087665684829'}) END