README.md states compatibility to 1.7.3+ but should be 1.8+

[gmolop]

PHP Fatal error: Uncaught ArgumentCountError: Too few arguments to function amp_ajax_button_check()

yourls_table_add_row function introduce second argument $keyword in v1.8, therefore plugin will throw error on previous versions.

https://github.com/YOURLS/YOURLS/blob/1.7.3/includes/functions-html.php#L560
$actions = yourls_apply_filter( 'table_add_row_action_array', $actions );
https://github.com/YOURLS/YOURLS/blob/1.8/includes/functions-html.php#L567
$actions = yourls_apply_filter( 'table_add_row_action_array', $actions, $keyword );

Quick compatibility fix will be to redefine the function amp_ajax_button_check as follow:

function amp_ajax_button_check( $actions, $keyword = null ) {
    // fallback compatibility prior v1.8
    if (is_null($keyword)) global $keyword;
    // define the amp capabilities that map to the buttons
    ...
}

But I think is better to update readme just in case something else cause problems.