How can i use internal base clock when i count two inputs

Hi. I'm using a USB-6343 and there is a internal base clocks.

I want to count two digital inputs and see in array, so i build a vi like below

But it says "External sample clock source must be specified for this application."

isn't this board(USB-6343) support internal base clock, or do i miss understand about internal base clock, or do i miss something?

should i use a external clock for a hardware count?