Use add_block('built-in/RateTransition', ...) without the white space.
This could be improved on the TMW side. If you look at doc add_block, it says:
You can use 'built-in/blocktype' as a source block path for Simulink built-in blocks (blocks available in Simulink block libraries that are not masked blocks), where blocktype is the built-in block's type, i.e., the value of its BlockType parameter.
The 'BlockType' property of that block is 'RateTransition', not 'Rate Transition'.
To use 'simulink/Signal Attributes/Rate Transition', you have to use char(10) to replace the white space in 'Signal Attributes'. This is another place that needs to be improved on the TMW side, in my opinion.