How to Import an Oracle dump version 5.1 into 12c?
Of course Oracle did not make this information public, but if you can access MOS, then go ahead:

Export/Import DataPump Parameter VERSION - Compatibility of Data Pump Between Different Oracle Versions (Doc ID 553337.1)

12.1 uses dump file sets of version 4.1. 12.2 uses dump file sets of version 5.1

You can not import a dump file created in 12.2 to 12.1, unless the VERSION parameter was specified to allow lower versions to import it.

You will need at least a 12.2 version database to import that dumpfile.