Tried FLAMINGJEZ solution but now my project has got bricked for modifying the script.

ScriptableSingleton already exists. Did you query the singleton in a constructor?
UnityEditor.PackageManager.UI.Internal.PackageManagerProjectSettings:.ctor ()

Restarting fixes the error message, but now some parts of my game wont work.

MELZERIA: You need to clear the error from the console before making the changes in visual studio, for some reason, and then it should work.

I tried your solution to fix this but everything I add the else and click on save, the script just automatically reverts back to what it had before and I keep getting the error message.

Thanks FLAMINGJEZ, It totally worked and I learned something new that I could've never had!

I fixed this myself as it was quite annoying. if you double click the error it should take you into the TerrainToolboxUtilities.cs. The error is that on line 1853 it's trying to call DisabledKeyword when m_PreviewMaterial is null. There's already an if statement checking if it's null, so i just added an else so the line only runs when m_PreviewMaterial has been assigned.

internal void RevertPreviewMaterial()
{
if (m_PreviewMaterial == null)
{
GetAndSetActiveRenderPipelineSettings();
}
else
{
m_PreviewMaterial.DisableKeyword("_SPLATMAP_PREVIEW");
}
for (int i = 0; i < m_Terrains.Length; i++)