Hi,
'INT' is the shortcut for "integer". So you want to use "string" instead as the canonical URL field is a text field.
And I'm sorry but I don't understand what you mean with your sentence
Also can I just check that doing this mass action - then disable this action - then turn canonical urls back on - entering the url for the contact form on the relevant catorgies/products - this would work with 404 pages?
What is the link between enter the link to a contact form page as canonical URL of the products and "work with 404 pages" ?
What does "work with 404 pages" means ? You want to have 404 pages ?