OK, its part of fcl-net, and Lazarus knows where to find it.
The error message, then, probably indicates that the compiler does not like your fpSock.ppu.
Perhaps it is missing?Or you have two conflicting copies with the same name in the unit paths?Or it is corrupt (or an older version)?
If you know how to rebuild fpc clean, you could try that.
Or you may need to uninstall and reinstall fpc.
Or someone else who knows how to deal with troublesome .ppus may have a less drastic suggestion.