Download is not working

If I hit the “download customers email list” button nothing happens. In console, I saw this error:

Also if you could tell me how to see more then 15 lines of customers on one page, that would be just perfect.

`mypage.com/wp-admin/admin-ajax.php 500
(anonymous) @ VM892:1
send @ load-scripts.php?c=0…lupload&ver=5.2.6:4
ajax @ load-scripts.php?c=0…lupload&ver=5.2.6:4
n.<computed> @ load-scripts.php?c=0…lupload&ver=5.2.6:4
(anonymous) @ custom-admin.js?ver=2.1:48
dispatch @ load-scripts.php?c=0…lupload&ver=5.2.6:3
r.handle @ load-scripts.php?c=0…lupload&ver=5.2.6:3