Yes, unless you don’t want it to be installed correctly and filter the way it’s supposed to! A canister filter must always be placed directly below the aquarium, because they require the force of gravity in order to properly function.
Depending on how tall your tank is, you might be able to position the canister beside the aquarium, rather than underneath it, but you’ll find that this significantly weakens the filter’s ability to siphon water, rendering it pretty much useless for the intended function.
It doesn’t necessarily need to be an eyesore, however. You can hide it in a cupboard underneath the tank, provided all of the tubing and the power cord is able to come through the back of the unit somehow and then fed into the water.
Don’t try and experiment with its positioning either. Standing upright vertically is the best way to ensure the filter functions as it is supposed to, and that your fish will have the nicest environment possible.
Although you can technically lay a canister filter onto its side and it will still function, this will cause the internal filtration system to be less successful, and could cause random contaminants to stay in your water as they sink to the “bottom” of the canister, which is actually its side.
Plus, the components inside the filter will have to work harder in order to function properly, which means they will wear out and break down faster, which means you’ll probably need to buy yourself a new filtration system way sooner than planned.
Given that they can be pretty expensive purchases, you want yours to last as long as possible. Therefore, you should always use it as the manufacturer recommends, and never stray from their instructions unless you want to void any warranty!