Could you please check and confirm which version of Lightroom are you using?
Which operating system are you using?
Also, please check you computer for the same font
(In Windows) Navigate to C:\Windows\Fonts and check for Mistral Regular
(In Mac) Check you font book.
I'm using Lightroom 6, in a Mac El Capitan
Mistral doesn't come with Lightroom, but I think it is installed with Microsoft Office.
So could it be that you used to work on a computer that had Office installed, but the one you're using now doesn't have Office?
Please check Font book on your machine for the font.
If the font is not available, Lightroom will not be able to provide it to you to use.
If it is unavailable, please download and install the font.
I am also missing mistral, i used to use it on PC but cannot find it on Mac, did you have any luck finding it?