If you give an immediate value to a Constant block, you cannot set the "unit" attribute to that value. If you want to set the "unit" attribute, use the Simulink.Parameter object.
I created a sample model with R2020a. You should open this model and update the model (Ctrl + D). Then you can see that the model has "units" set. At this time, set the output data type of the Constant block to Inherit: Inherit from'Constant value'.