"No such column on entity" error for an existing custom field

Make sure that this field is actually visible to your user via Field Level Security.

1. Click the field name in your Setup UI to view the field definition detail.
2. On this detail view, there will be a Set Field-Level Security button.
3. Make sure Visible is selected for your Profile.

This is usually if the system administrator profile does not have access to Read access to the fields .

I would recommend checking the Field Level Security for the System Administrator Profile for the field Course_Link__c