The UCC21520 is an isolated driver, and both output channels must be connected to independent power supplies. The lower MOSFET must be soldered in place—only then can the ground (VSSA) of Channel A connect to your system GND through the lower MOSFET when it’s turned on, forming a closed loop to charge the bootstrap capacitor for the high-side MOSFET. Then, when the lower MOSFET turns off and the upper MOSFET turns on, the high-side driver is powered via the bootstrap capacitor (note: the oscilloscope ground must be connected to VSSA).
Additionally, the two channels (A and B) must receive complementary PWM signals, and dead-time must be properly set.