This is a problem when transitioning non-SRP projects with terrains to SRP.
1) You have a non-SRP project with a terrain that has splat textures.
2) Export the terrain with its textures to an asset package.
3) Open your SRP project.
4) Import the terrain-and-textures assets.
5) Edit -> Render Pipeline -> Upgrade -> Lightweight -> Upgrade Project Materials
6) The terrain remains hot pink indicating it cannot find the materials.

You can recover by assigning an SRP-capable standard shader material as the terrain's default material. Alternatively you can delete the Settings/Lightweight_RenderPipeline but I suspect this might mess up other aspects of your SRP project.