Can't save grass

Hi newb here. I bought a grass field pack and placed some test grass using the foliage mode. I made a nice field and when I clicked on the floppy disk icon to save I got an error message saying.

Error saving’./././././././Users/PC User/Documents/unreal projects/MyProject6y/Content/StarterContent/Maps/Minimal_Default.umap’

I closed the project.

Opened it again and the grass field has gone, though the grass assets are still in the content browser, the ‘+ drop foliage here’ box is empty.

I know I’ve done, or am doing something wrong, can anyone help me by telling me what it is.