Verify the file has a valid risk type. Only exposed documents with a risk type are eligible for quarantine with the move option. Risk types include External DLP and Threat Protection such as Malware or VBA Macros.
This is also a requirement in a CASB policy where preserve Content includes Move.
This does not affect DLP's ability to issue a response rule to quarantine with tombstone.
Feature request ISFR-2446 was created to request that all files regardless of exposure type should be eligible to quarantine within CASB.