An ammeter is designed to be connected in series so the current passes through it. Since you don't want to change the original circuit, it's best if the ammeter has a very low resistance. Adding a very low resistance in series means the current won't decrease that much and the low resistance takes a small share of the voltage
If you connect an ammeter in parallel then there are two problems.
The first is that you haven't put the ammeter in the way of the current you're trying to measure so you can't be measuring it properly.
The second is that the current drawn increases so you're changing
the current you're trying to measure. A low resistance in parallel with a high resistance has an effective resistance of a little less than the low resistance.
The low resistance ammeter makes the effective resistance of the circuit very low and so the current is very big. The ammeter actually shorts out
the component it's trying to measure the current through.
This can damage the ammeter because a very big current flows in it.