We cannot save your changes because a site member has ended your editing session
What causes it and how to fix it:
- Creating a modern page via REST is a three-step dance, not one POSTCanvasContent1 won't stick on create; create → SavePageAsDraft → Publish, canvas is JSON; plain JSON is enough, Description needs its own write and a re-publish; the file name is random — rename with movetoCause and fix →