14.7 PlanAhead - [Constraints 18-5] Cannot loc terminal "XX_n" at site XX; loc is blocked

So this is a pretty annoying error. The solution is detailed

here:AR# 43315

and here also discusses it: Xilinx Forums

Under the [Project Manager] on the Left side of plan ahead, go to "Implementation Settings"

Under the Section for [Translate (ngdbuild)]  go to "More Options*" and type in:

-uc filelocation

^ use forward slashes / for the location like C:/Users/Xilinx/system.ucf

You MUST remove the constraints file from the project too to get the warnings to disappear.