You subscribe to extensions from the Extensions card in Licence and plans: Extensions. If something goes wrong, the notice begins Could not subscribe to "…" with the extension's name, and the line underneath says why.
The licence comes first
An extension is added to your JUDLEX licence, so without an active licence you cannot subscribe to one. The card says so with You need an active licence, or the notice does with Your licence is not active: take out JUDLEX before adding extensions to it. Activate or renew the licence first: The licence will not activate or has stopped working.
If the card says No connection to the server, JUDLEX could not reach our server. Check the connection and open Licence and plans again.
You cannot subscribe
This extension is already on your licence. There is nothing to do: you already have it.
This extension is for the Professional licence. It cannot be added to a Personal licence. If you need it, move to Professional first: Personal and Professional.
This extension cannot be bought, because it is included in your licence or is not on sale yet. Those still to come are marked In preparation.
"Subscribed, but we cannot deliver it"
The extension is already subscribed, but we cannot deliver its file at the moment. The notice puts it in capitals: do not press subscribe again. It is already active, and pressing again will not make it arrive sooner.
Let us know from Support: Getting help. It will be installed as soon as it can be delivered, with no second charge.
"Subscribed, but not finished on this computer"
The extension is subscribed, but it did not finish setting up on this computer: it could not be downloaded or could not be switched on.
Go back to the Extensions card and press it again, using Download on this computer if that button appears. Anything already subscribed is delivered without charging again. If it keeps happening, write to us.
If the notice mentions writing to the disk, check that you have free space and press again.
"This build cannot install extensions"
This copy of JUDLEX cannot check where an extension comes from, and without that check it installs none. Nothing has been subscribed or charged.
Install JUDLEX from judlex.com over the copy you have: Install and open JUDLEX. Your work is not touched.
The extension needs a newer JUDLEX
The notice reads That extension needs JUDLEX … and gives the version you have. The extension stays subscribed.
Update JUDLEX (Update now in the Notifications bell, or Update app in Your space) and press the extension again: Updates and downloads.
The prices shown are those of your version
If the notice begins The server is not selling extensions yet, or says that the prices you see are those of the last installed version, JUDLEX could not fetch the prices from our server and is showing the ones that came with your version. It is not your doing. Check again later: the amount you are charged is always the one the payment screen shows you before you confirm.