In fact you are using MLAG towards the Edge-Switches you don`t need to config STP, because it is also not recommended to use STP within a MLAG infrastructure. You can use STP (or any Loop-Protection) locally on your Edge-Switches to prevent loops.
Why do you want to add STP to this design? MLAG takes care of the loops between core and edge. The only reason to apply STP here would be edge port loop protection. So you can enable edge port mode with BPDU restrict on the edge ports.