mpbm23

The return URL has nothing to do with the redirect. Check your PHP version 5.6<. PHP version 7 is no supported.
If nothing works you must debug manually.

Papaki seams to have forked the payment Gateway so contact them directly for support.

Probably it can support but will not be added as also no further updates will be released.

Since this is a notice only and not an error you should disable debug from wp-config.php
define( 'WP_DEBUG', false );
The plugin will be working properly.
No updates will be released, from now on.

Since this is a notice only and not an error you should disable debug from wp-config.php
define( 'WP_DEBUG', false );
And probably the plugin will be working properly.

No updates will be provided for the latest versions of Woocommerce. You can revert to previous woocoomerce version.

Try using php 5.5 and not 7 I have not tested php7. Maybe something is different with the libraries.

Συν?βει και σε ?λλε? παραγγελ?ε? απ? τ?τε;

Is this still a problem? Did you have latest php and soap in the server? Does this exist in other installations?

Δεδομ?νου ?τι η μεθοδο? ε?ναι GET πρ?πει να επιστρ?φει και τα υπ?λοιπα στοιχε?α ?πω? γρ?φει?

Do you have more details about it?
What message does it show to you? It does not redirect you to viva?

You have checked preauth payments in the plugin options

Προ-?γκριση
Ενεργοποι?στε για να συλλ?γεται προεγκεκριμ?νε? πληρωμ??

Ο προεπιλεγμ?νο? τρ?πο? πληρωμ?? ε?ναι Αγορ?, ενεργοποι?στε για Προ-?γκριση πληρωμ?ν. Στη συν?χεια θα πρ?πει να αποδ?χεστε τι? πληρωμ?? μ?σα απ? το Admin Tool τη? Τρ?πεζα? Πειραι??

Uncheck to receive payments without preauth.

This is just a notice, you can prevent it by setting WP_DEBUG to false in your wp-config.php

You did not set properly the return urls
Success page : http(s)://yourdomain.gr/wc-api/WC_Piraeusbank_Gateway?peiraeus=success
Failure page : http(s)://yourdomain.gr/wc-api/WC_Piraeusbank_Gateway?peiraeus=fail
Cancel page : http(s)://yourdomain.gr/wc-api/WC_Piraeusbank_Gateway?peiraeus=cancel

If you have been redirected to the Piraeus Bank environment nothing needs to be changed in the code.
You just need to provide credit card number, CVV and change the months in the expiration date.