Here's how to fix the following problem with PrestaShop 1.7.x:
[PHP Warning #2] count(): Parameter must be an array or an object that implements Countable
( /classes/webservice/WebserviceOutputJSON.php, line 153)" ]
Log into the server, where your PrestaShop is installed (eg through FTP) and locate the file: /classes/webservice/WebserviceOutputJSON.php
And then find places where count is used.
The solution is to put @ before the count.
It tells PHP to validate the expression before evaluating it.
Change
..count(...
to
..@count(...
