The sign says "Customer Parking Only".
To be a customer I'm sure you agree that you need to enter the shop as a minimum.
So, enter the shop and buy something if you want to. If they haven't got what you want, or you don't see anything you fancy, or you've just gone in for five minutes to get out of the cold (in which case your status as a customer could be challenged but I doubt anyone will bother), and you cannot enter your VRM because you haven't got a receipt, see a member of staff and I'm sure they will help you out. Either that or pick up one of the many receipts that will be lying on the floor by the self-serve checkouts (some retailers provide a bin to put them in so you might be able to take one from there).
The parking conditions make no requirement to make a purchase. You have assumed that you must but you don't. Lidl's will make arrangements for you to enter your VRM if you are unable to.
Job Done!

PS - I would argue that the parking is free even if you have to make a purchase. Most Sainsbury's work on that principle. You pay to park (maximum 2 hours) and get a refund as you checkout (Minimum £5 spend in my local branch). You have to spend a fiver, but when you do the parking is free.