Audio interface problem

Hi, I am an unprofessional musician and try to record instruments in a hand-made home studio.

I am using;

Early 2011 MacBook Pro, 13-inch.

For the recording software, I use Garageband and Logic.

For recording equipment, I have Focusrite Scarlett 18i20 and it connect via USB 2.0

Here is my problem.

I get the exact same sound echoing when I plug my guitar and monitor through Garageband or Logic.

And the echo sound fades out after 2 seconds.

Getting this problem everytime I plug my guitar.

I don't think it is the audio interface's problem, I tried on built-in monitor of my interface, it worked fine.

Are there any solutions to fix this problem?

Thank you.