In the wordpress dashboard, it shows: An updated version of WordPress is available. You cannot update because WordPress 6.1 requires PHP version 5.6.20 or higher. You are running version 5.4.45.
This problem causes that my new theme can not be activated. Is there any solution for it?
This is a server problem, it is nothing to do with WordPress.
Please contact your host to upgrade your PHP version, they will help you update your hosting php version.