Let us check first Error Description:

ORA-04021 indicates that a session has attempted to acquire a lock on a resource, such as a table or a row, but was unable to do so within the specified timeout period.

One of my colluage got this error during export process on 19c database.

Actually you can get what is exact error from message. The problem related with locked object.

The question is here why export process hang and there is no error at alertlog. After making some search we can find easly to answer

This Oracle database setup is RAC, but during expdp commands CLUSTER=N parameter has been Forgotten. After put that parameter issue resolved