We can do anything with the amplifier, but the problem is almost always the computer. You said the computer had a Line-In. The web site did not say the computer had a line-in. Having one is critical for recording your music.
Most laptops have a place to plug in a mono, very low-level microphone for Skype or personal recording. Most Macs and Large Deskside PCs have a Stereo Line-In to record your amp.
So dig some more. It has to say in perfect clear English that you have a Stereo Line-In – or the socket on the side of the computer should be blue instead of pink.
Given that you don’t, you’re a candidate for one of the nice USB external Sound Cards out there.
https://forum.audacityteam.org/t/sound-card-reviews/8375/1
Koz