anyone know how to fix this bug or what causes it?
"Will not work in other case sensitive.."
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Far as I can tell it indicates it is finding a file with that name but with a case mismatch so you might have renamed it to fWalk.png or something.
note that yellow color indicates a warning, error would be red. So it's warning you that should you export to a platform that is case-sensitive this would become an issue.
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My practice to use only lowercase letters, numerals and underscore (_) in Godot file names. That avoids filename-related issues when exporting projects. Non-ASCII characters that depend on Unicode, such as ü, should be avoided.
Systems like linux (and probably mac, I don't use them) require filenames to have exactly the correct case. To them, Fwalk2.png.import and FWalk2.png.import are completely different (and incompatible) names. So you need to make sure the names of the files match the names to try to access them with.
Windows remembers the case of filenames, but ignores the case when opening a file. So this would run fine on windows, but fail on linux.