If your getting that error its because you didn't install everything in the proper order.
If you already have everything installed I suggest you completely uninstall PR and BF2 entirely. After you've done that reinstall BF2 and upgrade it to 1.41. Once that's done upgrade it again to 1.50. The next steps would be to install the core setup of PR followed by the patch followed by the levels.
Here's the URL for the BF2 patches.
Battlefield Blog
-Download v1.41 and v1.50
Here's the URL for the Core setup, levels setup, and patches for PR.
Project Reality : Battlefield 2 Mod - Downloads
-Download Client(Patch), Client(Core), Client(Levels)
Okay so after you've finished downloading all that, install BF2. Upgrade it to 1.41 and then 1.50. 'AFTER' you've finished installing and upgrading BF2 install PR using the 'core' setup file. Once you've finished installing PR via the core setup, you need to upgrade it using the patch file followed by installing the levels file.
After you finish doing all that your good to go. =]