Payment notices always say whether anything has been charged. When one says Nothing has been charged, there is no charge and you can try again without any risk of paying twice. How buying and paying work is explained in The licence.
The bank declined the card
The notice reads The bank did not accept the card, The bank declined the payment or Your bank declined the charge, and in every case Nothing has been charged. Your licence is exactly as it was.
Try another card, or change the one on your licence in Profile, under Payment method, with Change: Payment method. Then try again.
If you closed the payment page without paying, the notice reads You left the payment page without paying. Nothing has been charged. Start the purchase again whenever you like.
The payment is being confirmed
The notice reads Your payment is being confirmed, or says that a licence payment is already under way for this account.
A payment is waiting for the bank to confirm it. Do not pay again. Wait a few seconds or a few minutes: once the bank confirms it, the licence activates on its own. If you paid in your browser and JUDLEX is still waiting, I've already paid, check it looks again.
The account already has a licence
The notice says that this account already has an active licence. There is nothing to pay: a second licence is never charged.
What is missing is activating it on this computer. In Licence and plans, click Activate this computer or Find my licence. If what you want is more seats, they are changed at judlex.com, under My licences.
The amount has changed
The notice reads The amount has changed since you saw it or The amount you saw has expired, together with Nothing has been charged.
The amount you were shown is no longer valid, either because some time has passed or because something has changed. Ask for the amount again and confirm it. For extensions, JUDLEX works it out again by itself.
There is an unfinished payment
The notice reads There is an unfinished payment on your licence, or A payment for this licence is already in progress. Wait for it to finish.
An earlier charge on your licence is waiting for the bank, for example a card check that was left half done. You get two buttons:
Open the payment page, to finish it.
Cancel that payment, to withdraw it.
Then make the change you wanted again.
The server cannot take payment yet
The notice reads The server cannot take payment yet or The server is missing configuration for this change, together with Nothing has been charged.
It is not your card or your account: we cannot take that payment at the moment, and nothing has been charged. Let us know from Support: Getting help.
A similar case is Your organisation could not be checked. Try again in a moment. Nothing has been charged here either; try again after a little while.
Only the owner can change the licence
If the licence belongs to an organisation, only its owner can change the seats, the level and the extensions. The notice says that only the owner of the organisation, or of the licence, can change it. Ask that person to do it.