Are there any cases that Source Transformation method can not be applied?
My question here, Is there any Cases in Deducing Norton and Thevenin equivalents where I cannot use source transformation method?



There is no Thevenin equivalent for an ideal current source.
There is no Norton equivalent for an ideal voltage source.


Any other linear one-port network (meaning a network of linear resistors, ideal voltage and current sources, and linear dependent sources) will have an I-V curve from which either a Thevenin or Norton equivalent circuit can be found.