Problem: The encrypt wallet feature is disabled on cryptocurrencies that use zero knowledge proofs.
Solution: Until zk-snark technology matures, wallet encryption remains disabled due to its interference with z_addresses. There are workarounds such as using Third-party Veracrypt or other encryption software to keep your wallet.dat files safe.
Z-cash statement regarding problems with encrypting wallets with zk-snarks enabled : https://github.com/zcash/zcash/blob/master/doc/security-warnings.md#wallet-encryption
Original post regarding the issue: https://github.com/zcash/zcash/issues/1552