1. Loosen the axle nuts on the rear wheel.
2. Center the wheel in the frame and check tension again. Its easiest to center a wheel by measuring gap between tire and frame on both sides. If the tension is still off, adjust the tensioner on the right side using a 4mm allen key. Clockwise is to increase tension and counter clockwise loosens tension. Go in small increments and make sure the wheel is centered in the frame when you check tension.
3. Once you have reached the desired belt tension (1/2-3/4 inch deflection) tighten the right side axle nut. Check the tension again to make sure it didnt change.
4 Center the rear wheel in the frame by measuring frame to tire gap on both sides. Tighten the left side axle nut.
5. Pump your right brake lever 5 to 10 times and the brake should not rub as the wheel spins. If it does, adjust the rear caliper. https://www.parktool.com/blog/repair-help/hydraulic-disc-brake-alignment