Hi,
First you have to download SP Page Builder Pro from here
https://www.joomshaper.com/downloads/extension
Then you have to install it. Go to administrator-->extension-->install
After that you have to active it.Input your email and license key to SP Page Builder -> Pages -> Option (on the right) -> License & Update (input your email and license key) -> Save and Close.Your License Key is located in your Dashboard.
Then Clear Joomla and Browser cache.
By the way which template you are using?It is strongly recommended to not upgrade to the latest Joomla versions right now if you already have a live site. There can be issues. However, if you only have Helix Ultimate framework on your site, you can upgrade it to Helix Ultimate 2.x.x. This is not recommended if you have other templates as well in your site.
Our team is working on templates Joomla 4 compatible. Only Helix amd 16 more templates compatible now. Others will be compatible soon.You can follow this
https://www.joomshaper.com/blog/5-joomla-templates-updated-with-joomla-4
https://www.joomshaper.com/blog/11-joomla-templates-updated-with-joomla-4
-Regards