mod.io expects a zip file to be uploaded the zip needs to have a specific folder structure. You can automatically create the required zip by pressing the build zip button in the ModInfo data asset of your mod after cooking it.
This will gather all the required files and package them into a .zip using the provided 7zip executables in the mod kit.
The mod kit will automatically open the parent folder of the packaged build, once the zip creation is completed. Use this zip file in the next steps.