Html5 build error

I got the following error when I tried to build to html5. What is the cause?

The file names of your resources, or the name of the folder path in use, contains invalid characters.

The build/export system only supports a-z, 0-9, or hyphen and underscore characters. Anything else in folder paths will fail.

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Does that mean I can’t use capital letters either?

Capitals should be fine.