This is the easiest method.
You probably already have a user for which the link is not working for.
All you need to in Alma, is edit this user and change his account from external to internal.
You can easily toggle any user in Alma from being external to internal:
As an internal user clicking the LTI link will display an error screen including the LTI parameters.
In the error screen click the “View more” link to display the parameters.
This method will also work when using a user, that doesn’t exist at all in Alma.
How it looks like after clicking the link: