Steps to reproduce:

1. Create a new Unity project
2. Change Graphics API to Vulkan
3. Install the Terrain Tools package
4. Create a Terrain Gameobject (Create > 3D > Terrain) and select it
5. In the Inspector, click the "Paint Details"
6. Try painting terrain in the Scene view
7. Observe the Console

Actual results: 
{code:java}
ArgumentException: RenderTextureDesc graphicsFormat must be a supported GraphicsFormat. R8G8_SRGB is not supported on this platform.
Parameter name: desc.graphicsFormat
{code}
Expected results: No warnings or errors in the Console

Reproducible with versions: 5.3.0 (6000.0.50f1, 6000.1.6f1, 6000.2.0b4)

Also Reproduced on 6000.3.0a1 on Windows with Vulkan.

Tested on (OS): Ubuntu 22.04

Note: Not Reproducible with OpenGLCore