Fatal error after activation plugin

Hello! After activating the plugin I get an error on my test server:

Fatal error: Uncaught Error: Class 'losnappas\Ethpress\Plugin' not found in /var/www/httpdocs/wp-content/plugins/ethpress/ethpress.php:222 Stack trace: #0 /var/www/httpdocs/wp-settings.php(473): include_once() #1 /var/www/httpdocs/wp-config.php(110): require_once('/var/www/httpdo...') #2 /var/www/httpdocs/wp-load.php(50): require_once('/var/www/httpdo...') #3 /var/www/httpdocs/wp-blog-header.php(13): require_once('/var/www/httpdo...') #4 /var/www/httpdocs/index.php(17): require('/var/www/httpdo...') #5 {main} thrown in /var/www/httpdocs/wp-content/plugins/ethpress/ethpress.php on line 222 Notice: Function is_embed was called incorrectly. Conditional query tags do not work before the query is run. Before then, they always return false. Please see Debugging in WordPress for more information. (This message was added in version 3.1.0.) in /var/www/httpdocs/wp-includes/functions.php on line 6031 Notice: Function is_search was called incorrectly. Conditional query tags do not work before the query is run. Before then, they always return false. Please see Debugging in WordPress for more information. (This message was added in version 3.1.0.) in /var/www/httpdocs/wp-includes/functions.php on line 6031

I will be very grateful if you can help me to solve this problem. Thank you!