The online sandbox is an environment deployed as a Dynamics 365 Business Central service (SaaS). You can activate the online sandbox only from a production environment, by selecting Sandbox in the search box and clicking on the first choice (Sandbox Environment).
When the creation wizard starts, click Create:
A new browser window will open, and the sandbox creation process starts. At the end of this process, you have a welcome page that shows that the sandbox environment is ready. To start working with the online sandbox, click on the Close button:
When creating the online sandbox, a new Dynamics 365 Business Central tenant is deployed, and this tenant is created with a default demonstration company (CRONUS). At the moment, there isn't an option for copying data from the production tenant to the online sandbox.
You can reset your online sandbox environment...