Please keep in mind that the official Pixel Force support is no longer active.
A possible cause of this issue is that you didnt grant the "Storage" permission to the app Pixel Force.
To grant the "Storage" permission. Go to your system Settings > Apps > Pixel Force > Permissions (might be different for your device) and from there make sure the "Storage" permission is enabled, after you enable it, (re)start Pixel Force and try to find the directory again.
This is probably to prevent replaying the same custom level that has alot of money entites to gain easy money.
You can make a custom level for Pixel Force by downloading the program "Pixelforce Worldbuilder" from Digital Vaults website.
After you have your .lvl file somewhere on your phone device, copy it and paste in the following directory in your internal storage: pixelforce/lvls/myLevels.