error MSB6006: "cmd.exe" eixited with code 1.
It MIGHT BE because there are some operation in customer built steps failed. In project properties, go to customer step, you can see there maybe some something in command line edit box. Open a windows promote and paste the command to it. Run, check if there is something wrong and fix it.

If there is no command line in the project property custom built step, maybe you should check property of every single file of the project.

If the command line has some macro, replace it with an actual value.

Or you can echo the command in VS output window:

cd %(somedir)
echo %(somedir)

You won't miss it that way.