You did not post which X299 MB you have .... however I see you posted before about your Dark, so my answer is about the Dark X299 MB
 
 
USB 3.1 type C - p16, item #4 of the x299 Dark Manual   https://www.evga.com/support/manuals/files/151-SX-E299.pdf
 
p22
22. USB 3.0 Headers
 
"USB 3.1 Type-C (also found on the IO Hub) has a power limit of 3000mA (3A) @ 5V. USB 3.1 on the X299 Dark does not support USB PD protocol."
 
 
My cell phone's charger cable fits my X299 Dark (just checked it for you) - though the alignment is not perfect through the backplate - have to hold the cable slightly angled for it to slide in place
 
No idea why it would not fit before you mount the MB
 

dmann304
in the io area of USB, the C port seems to have no cable that fits it.  Even with board checked before 
installation in new case, i checked port to see if their was something stuck, nothing, so why doesn't
a standard type c cable not fit this port??  Is their a larger cable type that is made custom for this??

 

dmann304
Oh thanks very much. i was trying it before i installed in in new case awhile ago.  every time
i was trying to use port, cable didn't stay secured in place like it was lose.  Its like the port
is to big, or i need a longer head in C cable.

OK, when you said "did not fit" --> you where says it was too loose
 
Yes, my cable also is not a tight fit; the misalignment with I/O shield actually seems to hold it

Seems like in my case the I/O shield is just a tad higher than the MB USB 3.1 type C port & I need to angle the device to get it to insert as mentioned above