I deleted my site whose theme telnet was installed and on the new I can not install telnet it says it’s already on another, how I can disable it on a non-existent site
If the website is same mean same website but different domain then old license you can use but if the website is different mean different end product then author may require a new license.
To use purchase code (license) again which already used for any website (mean theme license already registered) you have to unregister/deactivate the theme license from the old domain/website. you can not use the same purchase code (license) for more than 1 website.
If you don’t have access of old website wp-admin where the theme registered/activated then you have to contact theme author and request them to unregister/deactivate the theme license from the old domain/website.
contact the theme Author and let them know How to Contact your purchased theme Author, the theme author will be happy to assist you within their scope.