WordPress Base Page not working?
I would suggest to try telling CiviCRM what is your base template.php pasting this in your functions.php

add_filter( 'civicrm_basepage_template', 'my_basepage_template' ); 

function my_basepage_template( $template ) { 
return 'page.php'; 
} 


Replace page.php with the page template in use by your theme.

If that doesn't work, try changing to the default WordPress theme (Twenty Fifteen) and try submitting the form in the Contribution page you have created and report back.