you need a custom firmware 3.xxm33.
if your firmware is 3.xx you have an official firmware and cannot play iso or run homebrew applications….
you need to install a custom firmware first then you will be able to play the iso.
look for a pandora battery (aka jigkick battery), it’s the easiest option and it will always be handy.
It installs the custom firmware for you in 2 minutes and also can unbrick your psp in case something wrong happens.

I’m not certain my answer applies to your question, but it’s a possibility.
ISO is a type of disk image. It’s possible you’ve just burned the compressed disk image to a CD rather than using a program like Nero to create the working copy of the disk.

I had this problem as well, Check that your custom firmware is installed properly and it is the newest version and check that the ISO is in the right place. If all fails it may be the ISO that does not work and you may need to rip the game and put it on again.
Also try using different ISO maker programs.
Cheers