Editor Won't Let Me Save My Main Level

I was moving my assets around into new folders, trying to organize them better. I tried to save my main map, and I get this error message:

Warning Can’t save G:/C Drive backup/Unreal Projects/SMash 4.12/Content/RollingBP/Maps/ROll.umap: Graph is linked to external private object Texture2D /Game/ROll.Texture2D_130 (MobileWeightNormalmapTexture)

I have no idea what this means. Please share your expertise!

Here’s how I solve it:
Settings → Preview Rendering Level → Mobile/HTML5 → iOS ES2 Preview
Then I can save it.