Recurring contributions on paypal pro - undefined index
I'm guessing you aren't using the most recent version of CiviCRM (4.7.27). In the current version of this file, that line has been replaced with:

$args['totalbillingcycles'] = CRM_Utils_Array::value('installments', $params);


The CRM_Utils_Array function is meant to prevent missing indices or variables that aren't even arrays from throwing warnings. (https://issues.civicrm.org/jira/browse/CRM-13288).

I have no idea when the change in the code happened (what version/date).

I suggest you upgrade your CiviCRM. It looks like it would be relatively harmless to update the code manually, in this case.