It won't let me make a copy/activate experimental gameplay in my world.

I have this massive building world. And I wanted to turn on experimental gameplay, it asked me to make a copy, so I clicked yes. And it made a copy. But it then said 'FAILED TO MAKE COPY'. But when I left Minecraft and re-opened the app, the copy was on my world page! BUT NO EXPERIMENTAL GAMEPLAY?!?! Please help!!