Hello,
I have test your html block “Menu home” which contains that mega menu. You are using a wrong link that’s why it leads you to 404 page.
This part “www.botanical-extracts.co.uk/” already included by default while using link on some image. But you again put that part that’s why the URL is not correct you just have to used this part “product-category/cbd-oil-cartridges/”:
You can check the last item in your mega menu which is shown on “Shop CBD” in your menu. I set an example for you.
Screenshot: https://jmp.sh/AZMJRiD
And while using the link in custom heading element. That list does not contain the product categories. So if you want to use a category then you have to put the whole link in the “URL” field.
Screenshot for clarification: https://jmp.sh/8GC3q6x
You can check the result by clicking this item image or below title, both will lead you to correct page: https://jmp.sh/QZowfQa
Best Regards.