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.
Right click and create a New Legend...
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".
This is where the first symbol will be placed with the tool.
Place a pure text next to the symbol. This is where the Description (Revit 2022-) or Fire_Info(Revit 2020-2021) will be placed.
Now run the Legend tool, and select the new legend template.