When I open my project in Easy FPS Editor It displays me this error:

And then I go to error.log and appears me this

Translation to English: The input string is not in the correct format.
Is there any way to fix that and avoid it? Thank you!
Notice that it says "their project," I think it's more of a question about trying to program something and encountering an error they don't know how to solve, and therefore it should go in General development
I'm not familiar with the Easy FPS Editor you mentioned, but the best thing to do is find its official community and ask there directly. You'll likely get help there than here.
Looking at the log, you're having a problem when the program tries to convert text to a number, and it's possible you're getting an error using a decimal point since English and Spanish use different symbols.