Use the Try and Buy mode if you want your existing customers to create trial sites even if Presence Builder is not provided with their license or they have exceeded the number of sites they can publish.
To configure the mode, update the settings of your hosting plans. To do that, open the settings and select the item Allow customer to create trial Presence Builder websites. After you update the plan settings, customers are able to launch Presence Builder by clicking the corresponding link in the Customer Panel.
This feature is controlled on a per plan basis, so you can offer the Try and Buy mode only for certain plans. If you deselect this item, your customers will still see the buttons to launch Presence Builder but they will not be able to actually launch it unless their plan includes Presence Builder support.
Note: If the hosting plans you want to change are imported from Business Manager and you want to keep them synced, go to the properties of corresponding plans in Business Manager, make sure that the item Allow up-selling Presence Builder sites is selected, and send changes to Plesk by clicking Events > Run Events.