Mitch Dale You’re close, the issue is that the Datastore query action doesn’t support functions like LOWER() in the way a full SQL engine would.
The way to handle case-insensitive matching here is to use LIKE, which is case-insensitive in this context. Try rewriting your query like this:
SELECT _id, Name, Telephone, ['VAT Number']
FROM AdsumClientList
WHERE Company LIKE '{{Company}}'
If you want a bit more flexibility, for example to ignore extra spaces or partial matches, you can also do:
WHERE Company LIKE '%{{Company}}%'