An ammeter is a device of a lower resistance value, since it is designed to measure the value of current in the circuit. So, it allows the current to pass through it, so we can obtain a reading. Now, if we connect an Ammeter in the parallel configuration, a large value of current would flow in the branch with the Ammeter (It’s all in the facts. Current will flow the path of least resistance). We can’t get an accurate measurement of the current in the circuit.
A voltmeter is a device that measures the amount of potential difference across a resistor or a series of resistors.