When accessing the Checklist Editor, you may get the error message: "app.checklist4sf.app doesn’t authorize the connection" or similar with an invalid link (sad document) icon. 

The cause is that users (including administrators) have to be explicitly granted access.

To fix this, go to

 > Setup.  

In the Lightning Setup, select 

> App Manager 

and for the row Checklist Editor select "Manage"

There are several option to grant users access.  The following grants access to users with certain Profiles.  Note that the user needs to have also the Permission Set 'Checklist Admin' and a license to see the apps.

As a first step, set the OAuth Policy:

- click on Edit Policies

- in the dialog under OAuth Policies, select "Admin approved users are pre-authorized" for the Permitted Users feld

- click Save

Then, in the Profiles section, click on Manage Profiles and select the profiles of users who should be able to enter/edit Checklists and click Save.

If you now go back to the Checklist Tab, you will be able to see and use the Checklist Editor.