Well, then it seems to be a known issue of the Commons extension:
As a workaround, you could try adding the Commons reconciliation service manually (for instance by opening a project unrelated to Wikimedia Commons).
Hopefully we'll publish a new version of this extension soon, so that people don't run into this problem anymore.
Amazing, thank you, that has absolutely fixed it, THANKYOU. I'd checked under "manage wikibase instances" but hadn't even thought to look through that route.