Hi @Proxima
Most probably you do not provide a correct input file format, that is why that option is grayed out! For surrogate tPAC, you need to provide the data itself (either in source or sensor domain -- the same data as what you've provide to compute tPAC itself initially). Also make sure you do NOT have the option of "average PAC maps ..." at the end of setting page marked for computing the original PAC and surrogate data.
It should help with your problem, if not let me know to check the problem in more details.
Best,
Soheila