I ran into the same issue on our webshop.
Looks like this is a typo in this file:
/wp-content/plugins/activecampaign-for-woocommerce/includes/abandoned_carts/class-activecampaign-for-woocommerce-run-abandonment-sync-command.php
The line 1026 reads:
$i += $diff->hours;
$diff seems to be of Type DateInterval, but according to php.net, DateInterval does not have a property “hours”. However it does have a property “h” which seems to be what we’re looking for in that case.
So for now you could fix this by changing the line above as follows:
$i += $diff->h;
Nonetheless the developer needs to fix it himself, otherwise we’re going to run into the same issue again when the plugin gets updated.