The add-in lets you sign in with a Microsoft 365 work or school account. When you have an account like that, the associated email address looks something like email@example.com or firstname.lastname@example.org.
If the only Microsoft account you have is a personal one (that is, hotmail.com, live.com, outlook.com, etc.) you can still try out parts of the Data Visualizer add-in without signing in. It just means that the features available to you are somewhat limited.
In order to be able to use all the features that come with the Data Visualizer add-in, you need to sign in with your Microsoft 365 work or school account.
If you don't have a Microsoft 365 work or school account
The features you can use when you're not signed in are:
Create a diagram from data in an Excel worksheet.
Update the diagram with your own data.
Print the diagram from Excel.
The features that are unavailable when you're not signed in are:
Open the diagram in Visio: to share the diagram, to export the diagram as an image, to export the diagram as a PDF, etc.
Print the diagram from Visio.
For further custom edits such as theme/layout changes, creating a diagram from your own custom stencils, adding images to org charts, and so on, a Visio license would be required.