Call to undefined function sqlsrv_connect()

I am currently testing this plugin for a project and am attempting to test a connection to a sandbox version of our MSSQL database. When I do so, I receive a 500 error with the following error log entries. I checked the files in the external-login plugin folder for sqlsrv_connect and it only finds the function call, not the function definition. Any help you can provide would be appreciated.

[11-Jun-2020 20:52:58 UTC] PHP Fatal error: Uncaught Error: Call to undefined function sqlsrv_connect() in /var/www/vhosts/immunolytics.com/imu2020.onecreative.net/wp-content/plugins/external-login/login/db.php:21
Stack trace:
#0 /var/www/vhosts/immunolytics.com/imu2020.onecreative.net/wp-content/plugins/external-login/login/db.php(225): exlog_get_external_db_instance_and_fields(‘mssql’)
#1 /var/www/vhosts/immunolytics.com/imu2020.onecreative.net/wp-content/plugins/external-login/options/testing_ajax.php(7): exlog_test_query(5)
#2 /var/www/vhosts/immunolytics.com/imu2020.onecreative.net/wp-includes/class-wp-hook.php(287): exlog_test_connection(”)
#3 /var/www/vhosts/immunolytics.com/imu2020.onecreative.net/wp-includes/class-wp-hook.php(311): WP_Hook->apply_filters(”, Array)
#4 /var/www/vhosts/immunolytics.com/imu2020.onecreative.net/wp-includes/plugin.php(478): WP_Hook->do_action(Array)
#5 /var/www/vhosts/immunolytics.com/imu2020.onecreative.net/wp-admin/admin-ajax.php(175): do_action(‘wp_ajax_exlog_t…’)
#6 {main}
thrown in /var/www/vhosts/immunolytics.com/imu2020.onecreative.net/wp-content/plugins/external-login/login/db.php on line 21
[11-Jun-2020 20:53:27 UTC] PHP Fatal error: Uncaught Error: Call to undefined function sqlsrv_connect() in /var/www/vhosts/immunolytics.com/imu2020.onecreative.net/wp-content/plugins/external-login/login/db.php:21
Stack trace:
#0 /var/www/vhosts/immunolytics.com/imu2020.onecreative.net/wp-content/plugins/external-login/login/db.php(225): exlog_get_external_db_instance_and_fields(‘mssql’)
#1 /var/www/vhosts/immunolytics.com/imu2020.onecreative.net/wp-content/plugins/external-login/options/testing_ajax.php(7): exlog_test_query(5)
#2 /var/www/vhosts/immunolytics.com/imu2020.onecreative.net/wp-includes/class-wp-hook.php(287): exlog_test_connection(”)
#3 /var/www/vhosts/immunolytics.com/imu2020.onecreative.net/wp-includes/class-wp-hook.php(311): WP_Hook->apply_filters(”, Array)
#4 /var/www/vhosts/immunolytics.com/imu2020.onecreative.net/wp-includes/plugin.php(478): WP_Hook->do_action(Array)
#5 /var/www/vhosts/immunolytics.com/imu2020.onecreative.net/wp-admin/admin-ajax.php(175): do_action(‘wp_ajax_exlog_t…’)
#6 {main}
thrown in /var/www/vhosts/immunolytics.com/imu2020.onecreative.net/wp-content/plugins/external-login/login/db.php on line 21