You're compiling the map from the esf maps folder? You don't have to do that, and I'd suggest making a folder to save your own maps in to keep everything organized, but it really doesn't matter.
It sounds like there is an error preceding this warning, and the error is stopping the generation of the .p0 file, which is why the compile tools can't access it.
Scan your compile log for errors before this message. If you don't see anything unusual, post your compile log, and hopefully one of us can find it.