Before Linked procedures can be used, the correct permissions need to be added to users. This needs to be done by a user with Admin access.
Select the gear on the main menu then select Account.
Then select Users.
To add a permission, select Update Permissions from the small gear on the right of the user's details.
There are three permissions for procedure linking,
Linked Procedures - Edit - allows a user to create, update and delete linked procedures under Settings/Procedure Links
Linked Procedures - View - allows a user to view the list of linked procedures under Settings/Procedure Links. No editing allowed.
Procedures - Linking - allows a user to add a linked procedure when adding new procedures under Procedures/Add Procedure.
The permission is added by clicking on it.