Error when using T1 fontenc and urw-garamond from mathdesign

You can get the needed fonts from CTAN.

Probably you need only the fonts themselves (the pfb-files). Put them in a local root in fonts/type1/garamond (see the MikTeX manual). Update the fndb and then try again.

I had the same problem, ended up here, and Will's answer pointed me to getnonfreefonts. That solved the problem for me. So, for anyone who stumbles here in the future, here is a quick answer:

(Linux)

wget https://tug.org/fonts/getnonfreefonts/install-getnonfreefonts
texlua install-getnonfreefonts

(macOS)

curl --remote-name https://tug.org/fonts/getnonfreefonts/install-getnonfreefonts
texlua install-getnonfreefonts

Then

getnonfreefonts -a

That installs all the fonts. If you want just want Garamond:

getnonfreefonts garamond

I think the problem is described in the package documentation for mathdesign:

This package alone is useless. You have to install one of the full set of fonts available. Please consult the provided README file. It deals with all the installation and system configuration process.

Do you have copies of the fonts required by mathdesign? (I think the script getnonfreefonts can help.) But from here I can't help you—I haven't installed these fonts before myself.