Common error in 4.9, one of many solutions:
1. use /set mod geta_challenge; after you type "geta" you can click TAB to autofinish.
2. put the file in a folder (can be new one, can be 'official mods' etc; and it'll work via modlist.
3. don't type whole name in searchbar, the error only appears if you type at least 2 letters, it shouldn't appear if you just type "g" and then scroll to the right mod.