If the project lacks the Legend template which is necessary for the legend tool to work. Follow the steps below to create a new template.

The template itself needs three things. A Detail Group, a legend component, and a text for descriptions.

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Right click and create a New Legend...

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Name the legend and leave the scale for now.

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Create a Reference plan, this is to be able to snap the viewport in the sheet.

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Create a text and write the title. Select the text and create a detail group. This will create a local coordinate system in the legend view, which is necessary for placing the symbols with the tool.

Name the group, i.e. "LegendGroup".

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Place a legend component

This is where the first symbol will be placed with the tool.

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Place a pure text next to the symbol. This is where the Description (Revit 2022-) or Fire_Info(Revit 2020-2021) will be placed.

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Now run the Legend tool, and select the new legend template.