Hello Aqilla:
The warning messages indicate that some source cells (the cells from which you start measuring accumulative cost) are not being considered because they are located on NODATA cost cells.
This isn't necessarily a problem, but you should investigate further. Can you provide screenshots of your input datasets and the parameters used for the gp tool?
By the way, if you are using a recent version of arcgis pro, i recommend switching to the Distance Accumulation tool. It has an improved algorithm which, in particular, can make a difference when routing shortest paths around barriers (groups of NODATA cells).
Jim TenBrink
spatial analyst team