You can unlock the FileVaultMaster keychain using command followed by providing password.
Unlock the FileVaultMaster keychain by entering the following command and enter the password you used when the keychain was created.
security unlock-keychain /Library/Keychains/FileVaultMaster.keychain
If you get an unexpected result during this step, the unlock idle time might have elapsed. You need to re-issue the unlock command in the Terminal window.