Hi,
1. This error happens when Xilinx ISE could not find the netlist for the related component. Suppose you have instantiated a component in your toplevel, and you forgot to source that file during synthesis. Then, when you give for place and route, ISE is not able to get that component and throws this error.
2. go to command prompt and change directory to where bgdbuild file situated (example : C:\xilinx\bin\nt), source your design for compilation.
Regards
Vs21