Sometimes You can get “ORA-12557: TNS:protocol adapter not loadable ” error.
Details of error are as follows.
ORA-12557: TNS:protocol adapter not loadable Cause: On some platforms (such as OS/2) protocol adapters are loaded at run-time. If the shared library (or DLL) for the protocol adapter is missing or one of its supporting libraries is missing then this error is returned. Action: For further details, turn on tracing and reexecute the operation. The trace file will include the name of the shared library (or DLL) that could not be loaded.
Check PATH variable in Windows
Check PATH variable in Linux/Unix
Or This error is caused by 32 bit application with 64 bit database, so try to use same client version for 64 bit database.
Or Check your Oracle Home and Oracle SID if they are set correctly or not.
Do you want to learn more details about RMAN, then read the following articles.