'Unable to add item'

The items have suddenly become unavailable when we try and add to the basket. It is saying 'unable to add item'. We have got the items listen with inventory so not sure how this can be resolved. Could anyone help? I will add screenshot. We have tried making a copy, making new products, adding and taking away / Nothing working!

The URL is currently under a test name. Any help you can give will be appreciated!!

https://www.chiswickhousedogshow.org.uk/class-registration-copy

Hello,

Did this issue get resolved? I am running into the same issues as I add new product to my site. 

Thanks