I just had this same problem with Payment Pro downloaded in June 2018.
I had this same problem with new install of Payment Pro when trying to activate PayPal Payments. I got a error code saying this:
Please, change the crypt key (PAYMENT_PRO_CRYPT_KEY) in /home/myserverplace/public_html/mydomainplace/oc-content/plugins/payment_pro/config.php.
Then it had a click link telling if the config.php file was not there to make that file adding this code to it, but change the password to something else with 24 digits:
<?php
if(!defined('PAYMENT_PRO_CRYPT_KEY')) {
define('PAYMENT_PRO_CRYPT_KEY', 'randompasswordchangethis');
}
So I made a new file with that and changed the password to another with 24 digits and uploaded to correct location. Then when attempted to install PayPal again, I got this message:
Warning: Cannot modify header information - headers already sent by (output started at /home/myserverplace/public_html/mydomainplace/oc-content/plugins/payment_pro/config.php:1) in /home/myserveplace/public_html/mydomainplace/oc-includes/osclass/utils.php on line 1743
I read in this form to make sure no white spaces above or after <?php and there was not. Then I read another post to backup my files/database and reinstall everything. Before I did that, I took one more look at the code the popup wanted me to install in the new config.php file......
<?php
if(!defined('PAYMENT_PRO_CRYPT_KEY')) {
define('PAYMENT_PRO_CRYPT_KEY', 'randompasswordchangethis');
}
And there it was! A little error that made a big headache for several of us who are no experts with php. It was in this line....
define('PAYMENT_PRO_CRYPT_KEY', 'randompasswordchangethis');
Notice there is an extra SPACE after the comma! I REMOVED THAT SPACE to read this:
define('PAYMENT_PRO_CRYPT_KEY','randompasswordchangethis');
Then went back in to finish the install of PayPal, and it worked!