If experiencing issues with missing materials, one may attempt to follow the steps below.

Causes:
One of the following: 
1. Autodesk Material Library isn't installed.
2. Rendering file path isn't set or isn't set correctly.

Solution:
Depending on the cause, proceed with the applicable solution:

"Autodesk Material Library (Base/Low/Medium) Resolution Image Library" isn't installed:
1. Check the Windows Programs/features list for "Autodesk Material Library <version.>" 
2. If not listed, follow the steps in this article "Revit Material Library did not install".

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"Autodesk Material Library (Base/Low/Medium) Resolution Image Library"  is installed, but material textures are missing (from C:\Program Files\Common Files\Autodesk Shared\Materials\Textures):
1. ​Repair (through the Programs and Features Control Panel) each of the Revit material libraries (Base/Low/Medium) to restore the missing Textures.
2. If repairing does not restore the textures, then uninstall the material libraries from the Programs and Features Control Panel, and reinstall using the article above.

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Rendering/file path isn't set or isn't set correctly:
1. Hover the mouse over the red area.
2. Note the file path which Revit is using to find the file/material.
3. Verify that this path is accessible and that the file is available within the path.
4. If the path is accessible and the file exists, repair Autodesk Material libraries in Programs and Features.

Note: If the file path is pointing to a network location, make sure that the path can be recognized by Revit. Avoid mapped letter drive, use UNC path instead.

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~ THE MOST MAINSTREAM AND EFFECTIVE METHOD ~

Add a new rendering path to Revit:
1. Open Revit Options.
2. Click Rendering > Add Value.
3. Insert the path to the "Materials" folder in the downloaded Revit file folder. 
For example: "C:\Download\ETH_FU_Bok_DiningChair\Materials." 

Note: Correct the path if not applicable to your material folder. 


Alternatively, copy the entire JPG materials in folder "Materials" and paste it to the existing Revit Material Library path (C:\Program Files\Common Files\Autodesk Shared\Materials\Textures).

Note: Correct the path if not applicable to your material folder. 

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source: https://help.autodesk.com/view/RVT/2025/ENU/?caas=caas/sfdcarticles/sfdcarticles/Revit-Material-Textures-are-not-accessible-visible-in-the-Material-Browser.html