I got the same error for cx_oracle (via python). For this specific case ORACLE_HOME was missing in environment variables.
Set Oracle_home with Windows Cli (use your oracle folder):
setx ORACLE_HOME "C:oracleoc121"
On some of the DB connections TNS_ADMIN is required also set that as well:
setx TNS_ADMIN "%ORACLE HOME%
etworkadmin"
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