Fonts are not TrueType
This check flags up when only TrueType fonts are used in the InDesign document.
TrueType fonts were developed by Apple and Microsoft as a cheaper alternative to Adobe PostScript. They remain a valid font format for print and digital outputs but are beginning to show their age a little. They can be OS-specific and have a lack of multilingual support.
Also, some production experts can be suspicious of TrueType fonts as they are less likely to have come from a professional font foundry or may be system fonts which can alter with OS changes.
These days TrueType has largely been superseded by the later OpenType format which is cross-platform (used on Mac and PC) and has full multilingual support, but may well be present in legacy / archived documents.
InDesign identifies TrueType using a ‘double T’ symbol:
You can also see which are TrueType in your document by choosing InDesign Type menu > Find/Replace Font and clicking the More Info button. TrueType fonts will be shown by their Type in the Info panel below:
Fix this problem by finding and activating a different font format in InDesign or in some cases by substituting for other fonts.
Adobe InDesign Font help
Apple FontBook help
