error of type E_PARSE

Hello there!
It looks like the refactored version has a syntax error?

?Timezone

Error Details
=============
An error of type E_PARSE was caused in line 18 of the file /var/www/html/public_html/wp-content/plugins/video-conferencing-with-zoom-api/includes/Timezone.php. Error message: syntax error, unexpected ‘?’, expecting function (T_FUNCTION) or const (T_CONST)

private static ?Timezone $_instance = null;