When you go into an amp input the first thing the signal does is to go through a Pre amp before getting sent to the amp.
If you're going into a line input on your interface, the interface assumes you've already gone through a pre amp stage.
Pre amp stages are generally found at the mic inputs in various mixers or interfaces.
Your pick up as well as microphones have too low of an output to be handled by Line In devices (which are looking for higher levels like keyboards, drum machines, etc)
Now, to complicate matters, most mic inputs are XLR while your pickup is probably 1/4". Some interfaces provide a 1/4" input to the preamp stage and is labeld DI input.
Look at the back of your gtr amp to see if there's a preamp out or even a send/return insert might do.
If you don't have this, your next best bet is to look into purchasing a Direct Box.
hope that helps

Some interfaces have a Mic/Line switch.

I was using preamp while using the acoustic pickup, but i also noticed that i had to turn the audio up by a lot. I'm probably going to have to purchase a direct box since i have a cheap 100$ interface and it doesn't have a DI input. Thanks Natural!!