The error "the value of a field does not conform to the expected format" may be shown on the payment page during checkout.
This message prevents the customer from terminate the purchase with PayPal.
The error is reported when the value of a field does not conform to the expected format, and the problem is often related to the URL in use for the payment page.
Often, The URL does not conform in that it has a space between characters.
In general, One must avoid spaces in a URL.
The same URL can be rewritten in this way:
Making this change, will solve the problem.
To proceed, in the PrestaShop menu click on "Shop parameters", then on "Traffic & SEO."
Check the Friendly URLs associated with the payment steps.
For example, that of the "Orders" page.
If a space is present in this field, edit the corresponding page by writing the new URL in the "Rewritten URL" field.
It is necessary to check the URLs of all the languages you have in your Back Office.
Save the change and checkout process again.