The full path of the source block is incorrect. The best way to handle this, is to do it manually.
If you are able to open the powertrain library manully, then select the block you want to add, go to MATLAB Command Window, type 'gcb'. It will show you the correct path of the block. Then you can use it in the add_block(). For example:
>> gcb
ans =
'autolibmotorctrlr/Interior PM Controller'
