Optocoupler Reference
When driving your IGBT bridge, the floating supply on the 'hot' side of your optoisolator must be totally unconnected to the low voltage side. The 'floating' means with respect to the input side.

You will need four isolated supplies, one for the HV -ve, and one for each phase.

Depending on the amount of power you need on the IGBT side, this could be done with a transformer with several isolated secondaries, each driving their own rectifier/capacitor, or you can buy isolated supply modules which use a transformer internally. For a quick test, you can even use batteries.

For the SCR, the 12 V supply GND must be connected to the SCR GND, and ket isolated from the MCU GND.