Why is the message appearing ' Sorry, we only have 1 of that item available', when this isn't a problem and not been set up to say this?!?

Site URL: https://www.sumofeight.com/

We have 1 of this item left to be purchased on our web shop. However, when on my phone, recently this message has appeared stating I cannot add the item to the cart as there is only 1 left. This should not appear as we want the last one to be sold, and we have not set this up (that we know of), but cannot find how to stop the message from appearing! Or as to why it appears

Any help as to what to do would be great, thank you 🙂

When I tried, I could add this item to the cart.

Is it possible you are trying to add a second one to your cart? Your cart icon shows one item in there already.