Please note you need an admin account to resolve this.
There are two likely reasons.
1. The custom domain isn't properly set up.
Use of Scan to Salesforce requires you to have a custom domain. Please go here for more information.
2. The custom tab isn't properly set up.
Go to the Salesforce dashboard and in the top right select the gear icon > Administration > Users > Profiles. Choose the profile the user has and set go to Custom Tab Settings. Turn Scan to Salesforce setting to "Default On".