VidSync can correct the radial (barrel) distortion caused by wide-angle or fisheye lenses. However, even lenses with minimal distortion to the naked eye (such as GoPros using an intermediate or narrow field-of-view) still have some amount of distortion that is highly worth modeling and correcting to maximize measurement accuracy.
Try to fill the frame with the checkerboard, so lines near the image edges can inform the distortion parameters. In the program, you will typically use automatic plumbline detection to digitize a large number of reference lines that are straight in the real world but slightly curved on the screen (and thereby give the algorithm a way to calculate and correct for that curvature). However, if automatic detection fails, you can manually digitize as many straight lines as are needed to obtain sensible results from the algorithm.
Try to perform the distortion correction on a frame in which the checkerboard fills the screen, or very nearly fills it. If the board is distant (occupying only the middle 1/2 or 2/3 of the screen) the distortion parameter estimates will be less consistent.