I can't find LINE IN - Audio Input Problem with a Mac Book pro early 2015
You don't have a line in.
Per the Technical Specifications for your MacBook, it has support for a line out, but not line in.

Audio

Stereo speakers
Dual microphones
3.5 mm headphone jack

Support for Apple iPhone headset with remote and microphone
Support for audio line out (digital/analog)




Emphasis mine
To get a Line-In, I suggest using a USB-DAC or a USB Mixer so you can get a proper line level input and output.  In fact, I try not to use any of the built in audio ports due to their poor shielding and &quot;economy grade&quot; components.  You'll get better results with a proper audio interface.