The basic syntax of ADD_BLOCK is add_block('src', 'dest'), which copies the block with the full path name that you specify with 'src' to a new block with a full path name that you specify with 'dest'.
The reason of the error is due to the different full path of the blocks in SimScape Library compared to those in Simulink library. Take Thermal Mass as an example, you could get the full path of the block by the following steps:
a) Open Simulink Library Browser and right click on Foundation library
b) Click 'Open Foundation Llibrary Library'
c) Find the Thermal Mass and left click it
d) Execute 'gcb' in MATLAB command window, you'll have 'fl_lib/Thermal/Thermal Elements/Thermal Mass', which is the correct 'src' value for adding Thermal Mass in your model with ADD_BLOCK.
