If that is direct burial cable (aka "mobile home feeder") then it can be direct buried, but as soon as it enters the building (slab, block, walls, whatever) it needs to be in properly installed conduit.
Is there conduit directly under that bottom plate? If there is, you need to chisel out enough wood to expose it and properly glue/join wide sweep elbows, etc to make the transition to the section going into the panelboard.
Make the bends in the panel as tight a radius as allowed for that size cable and get that cable as far out to the sides and bottom of the box as possible, otherwise they will be in the way of running your wires to the breakers.
Did you set the front of the panel out from the studs enough that it will be flush when you install the sheetrock/OSB/whatever wall cover you are using?
Charles