Make sure you have all the required credentials before configuring the plugin.
To do this, contact the IT contact at your GLS depot. The IT department at GLS has to activate you for a so-called Uni-Box at the depot. If the depot in question does not operate its own Uni-Box, ask for the public Uni-Box in the data centre for the respective region.
You also need parcel number ranges for all the products you use. When talking to GLS, please mention that you are using a standard product approved by GLS and not a customised solution - this helps to avoid misunderstandings.