Lots of things; probably fit, then likely pushing too-high resistance, too high cadence or probably all of the above.
Stationary bikes are usually direct-drive, so it’s like riding fixed- gear, if you’re not working against the bike, the bike is working against you, there’s no coasting. Lowering the resistance and turning a higher cadence will cut down on the push-and-pull that can beat up your legs.
I find that stationary bikes, especially non-interactive “dumb” bikes are like drinking non-alcoholic beer; it’s okay, but it’s missing something. Without the wheels on the road, changing gears, steering and handling the bike, it’s just not the same and I can’t convince myself otherwise.