Hi,
There was a time when the paypal module did not allow the customer to complete the purchase.
Had the version prestashop 1.7.7.1.
😟Error in Paypal:
The value of a field is either too short or too long.
I leave my help.
SOLUTION (how I solved it) 😊:
1) Please update the module to the version 5.4.4 and clear the cache.
2) Please, in your BO go to shop parameters>Traffic and SEO and check all your friendly urls. If you have there more than one word, make sure they're written with dash instead of space between them. F.e. not "the test" but "the-test". That should solve the issue.
If not, please check also your ISO codes (International-localization-country). They should be correct and valid.
Please pay attention that it is necessary to check the urls for all languages that you have in your Back Office!