If you're behind a proxy and want to use Zeplin's Photoshop plugin, open a text editor and type the following lines as necessary:
HTTP_PROXY=http://your proxy address
PROXY_USER=your username (This line is optional.)
PROXY_PASS=your password (This line is optional.)
Name the file plugin.env and save it to /Applications/Adobe Photoshop 2020/Plug-ins/Generator/io.zeplin.photoshop-plugin (Photoshop directory name might be different depending on the version you're using) then restart Photoshop.