Hi,
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The trial version is fully functional. There are two reasons why the build button may not be available:
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1. You have not placed a chart App Part on a page. Each chart instance is uniquely identified by it's web part ID. Web parts (app parts) are assigned a web part ID when they are added to a page. If you open the Chart App directly within Site Contents, then the chart app is not running within the context of a specific web part id, and so there is no actual chart to be built.
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2. You do not have the necessary permissions to configure the chart. From the documentation:
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To configure charts, you need Add and Customize Pages permissions (granted by assigning Design or Full Control rights).
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Please let us know if that does not solve your issue.