Hi @Thanushiyah,
The assert(freq != 0UL) line is either from the CLOCK_GetFreq() function, or the CLOCK_GetPllFreq() function.
These functions return the operation frequency of one of several clocks, depending on the argument that is passed onto them. If the frequency of the desired clock is not correctly set up, the function will return that assert error.
I would recommend debugging the application step by step to locate what argument are being passed to the function, and therefore better pinpoint what clock is not being enabled properly. From there, it's a simple matter of making sure the erroneous clock gets set properly.
BR,
Edwin.