Scheduled events are not possible in the Sandbox environment. That's by design as 1) it would consume resources which are provided free and 2) application goes to sleep if it is not used (so Scheduled event won't run).
Some details can be found here
Like Michal said, it is not possible to run Scheduled events in the sandbox accounts.
If you want to test it on the Sandbox environment, best you can do is create a test button that triggers the same microflow as the Scheduled event.
The sandbox environment is created and used when you click on ‘Run’ instead of ‘Run locally’, is this also what you are doing?