PrestaShop back-office can be accessed and used using various URL, for example : mywebsite.com and www.mywebsite.com. However, for technical reasons, eBay module will only work using default PrestaShop URL.
Below error message indicate you accessed to PrestaShop back-office not using default PrestaShop URL :
To access PrestaShop back-office using default PrestaShop URL, simply click on the link available in the error message.
PrestaShop in mono-shop mode
You can find default PrestaShop URL in “SEO & URLs” under the “Preferences” tab in your PrestaShop back-office.
PrestaShop in multi-shop mode
Keep in mind PrestaShop has a default shop URL for each shop in multi-shop mode. You can find default URL of each shop in “Advanced settings” > “Multishop”. When using module for multiple shops, you will need to change URL for each shop.