Ensar, this is a warning not an error.

I assume you have a large number of fonts installed on your PC and have made liberal use of them. Unfortunately, not everyone has those fonts, so when someone views your site (in the past) they would probably see Times Roman text.

With v15, Incomedia have taken steps to try and ensure only websafe fonts are used (fonts which will display correctly on everyone's PC, mobile, tablet, etc). As the message says, the non websafe fonts will be replaced by your default font (set here)

Rather than use local fonts, I strongly recommend using google fonts. You can add and/or browse for new fonts at any time.

And there are thousands to choose from