You can download most, but not all, of the fonts shared on FontStruct.
Some “FontStructors” (font designers on FontStruct) choose to share their designs but not to allow downloads; so you can see a preview of the font but you can't download it. One reason for this is that the designer has made the font for a commercial project, or that they sell the font on a different site.
If there is no Download button visible on a FontStruction page then you simply cannot download the font from FontStruct.
Please respect the designers' choice, and do not write comments asking users to change their license in order to allow downloads.
If you want to use the font for a specific project you can try contacting the designer and asking their permission.
Please contact us if you can’t find the information you’re looking for here.