How to connect battery pack with AC/DC out to marine amplifier with screw type terminal block connection?
The battery pack has an answered question on Amazon stating that the max draw is 3A:



The Kenwood amplifier lists 20A current draw.  Therefore, if you connect them, you will either A) blow a fuse inside the battery pack, or B) start a fire.

There are many larger batteries that are easily capable of sourcing 20A.  As you mentioned in a comment, a car battery should be able to handle the load.  You should be able to find a Lithium Ion battery that can handle it as well (one example is this one).  The important thing is making sure that the max continuous discharge current is at least 20A (preferably higher so that you have some margin).

Once you've picked the battery, it's just a matter of wiring the positive and negative terminals to your amp.  The AWG recommendations from https://www.powerstream.com/Wire_Size.htm indicate that you want AT LEAST 16AWG wire.

I'd recommend using spade or ring terminals rather than bare wire in the screw terminals - it will make the connection more reliable and less likely to break.