This could be due to a number of reasons;
Are you using the ""test server"" version of SnowRunner? The mod menus are sometimes incompatible with this version, please try switching to the main version of the game to use mod.io. Similarly, you must be using a legal (purchased) version of the game, with the latest updates applied, for the mod menus to work.
If you are using the main version of the game then typical fix for this issue is to force SnowRunner to reauthenticate;
1. Close SnowRunner
2. Browse to C:\Users\%USERNAME%\Documents\My Games\SnowRunner\base\Mods\.modio\
3. Delete the authentication.json file
4. Restart SnowRunner
5. Log back into the mod menu