Why export is showing bug?

I saw the following bug. What is the reason, @GeorgSeifert?

No glyph for a standard character to derive standard width and height.
Please check the documentation for a list of script-specific standard characters,
or use option `–symbol’.

Your ttfautohint options are incorrect. See the manual for explanations about the options.

Why is it incorrect? Also the problem in some new fonts.

That is stated the error message: ttfautohint needs a default character for each script that is employed. Otherwise it cannot measure and determine standard stems.

I do not know your design, but you are trying to use ttfautohint without giving it the information it needs. Are you sure it is a hintable font in the first place?