Why must a voltmeter be connected in parallel?

It has a very high resistance, so if it is in series the current in the circuit would be close to zero.

A voltmeter has to be connected in parallel as a voltmeter has high resistance so it doesn't affect the flow of the current passing through it if it's connected in parallel

A voltmeter is always built with an extremely high resistance.

From V=IR we can see that for a constant voltage increasing the resistance in a circuit will reduce the current.

Therefore if the voltmeter were to be put in series it would reduce the current in the circuit.

By placing the voltmeter in parallel across the component we want to measure we ensure that the voltmeter does not alter the current in the circuit.