Projects older then version 1.3 follow the guide linked below
https://defusestudios.com/rts-template-add-new-resource-types/

Open the E_ResourceNames from the Content Browser and add a new entry, name it the name of your new resource.

Next go to the BP_RTSPawn and copy the video below.

Next open the BP_HUDBuildingMenu and copy the video below.

Next open BP_HUDUnit and copy the video below.

Resources Actors

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The video below is an example of how to setup a new resource actor, this shows where you can set the resource mesh, min and max size of the mesh and how to place and setup the resource spawning volume.

Buildings and Units

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The video below is an example of how to setup a unit to collect the new resource and a building for resource drop off.