OJS3.1 Paypal function not showing after setting

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Hello.
I set up the paypal, and filled in all account name, ID, secret. but nothing changed. please advise me how to proceed.
error log shows as below:

[Fri Apr 20 00:40:13.913751 2018] [:error] [pid 27641] [client 89.230.196.85:55589] script ‘/var/www/html/ojs/wp-login.php’ not found or unable to stat
[Fri Apr 20 00:46:28.471182 2018] [:error] [pid 31717] [client 14.17.3.65:34645] ojs2: 404 Not Found
[Fri Apr 20 00:48:50.299732 2018] [:error] [pid 32181] [client 101.226.68.215:23861] ojs2: 404 Not Found
[Fri Apr 20 00:49:04.750011 2018] [:error] [pid 31712] [client 36.149.129.108:31313] PHP Notice: Array to string conversion in /var/www/html/ojs/lib/pkp/classes/submission/Submission.inc.php on line 408, referer: http://tspsubmission.com/index.php/CMES/submissions
[Fri Apr 20 00:49:04.750058 2018] [:error] [pid 31712] [client 36.149.129.108:31313] PHP Notice: Undefined variable: subtitle in /var/www/html/ojs/lib/pkp/classes/submission/Submission.inc.php on line 495, referer: http://tspsubmission.com/index.php/CMES/submissions
[Fri Apr 20 00:49:07.128601 2018] [:error] [pid 27981] [client 36.149.129.108:31314] PHP Notice: Array to string conversion in /var/www/html/ojs/lib/pkp/classes/submission/Submission.inc.php on line 408, referer: http://tspsubmission.com/index.php/CMES/submissions
[Fri Apr 20 00:49:07.128649 2018] [:error] [pid 27981] [client 36.149.129.108:31314] PHP Notice: Undefined variable: subtitle in /var/www/html/ojs/lib/pkp/classes/submission/Submission.inc.php on line 495, referer: http://tspsubmission.com/index.php/CMES/submissions
[Fri Apr 20 00:49:09.650885 2018] [:error] [pid 31716] [client 36.149.129.108:31315] PHP Notice: Array to string conversion in /var/www/html/ojs/lib/pkp/classes/submission/Submission.inc.php on line 408, referer: http://tspsubmission.com/index.php/CMES/submissions
[Fri Apr 20 00:49:09.650932 2018] [:error] [pid 31716] [client 36.149.129.108:31315] PHP Notice: Undefined variable: subtitle in /var/www/html/ojs/lib/pkp/classes/submission/Submission.inc.php on line 495, referer: http://tspsubmission.com/index.php/CMES/submissions
[Fri Apr 20 00:51:32.053773 2018] [:error] [pid 27981] [client 14.17.3.64:36357] ojs2: 404 Not Found
[Fri Apr 20 00:57:00.548483 2018] [:error] [pid 27261] [client 183.57.53.177:52485] ojs2: 404 Not Found

I set up the REST API, created a APP and get all the information.
I do no know how to activate the payment function.
@asmecher

Hi @mujiec,

Can you describe the problem in more detail? What part of the system are you working with, what do you expect to happen, and what happens instead?

Regards,
Alec Smecher
Public Knowledge Project Team

The problem is when I create the API key and insert it in the Setting–>Distribution–>Payment paypal setting.
I expected to show a payment option on the left menu.
What happened was it did not show and nothing changed. I do not know where to activate the payment function.
Thank you very much @asmecher

Please check the screenshots, I cannot find the payments or subscriptions options at the left menu, however, when I ender “subscriptions” at the end of the URL, it showed the subscriptions option page.
image

Please check, also I do not know how author will be charged. Is this payment function applied after the approval of article?
Thank you very much

@asmecher
Is there any plugin I need to install, the menu does not appear payment option.
Thank you.