InvalidOperationException

Hello all,

I’m new to using unity and I’m try to make a map for a game called VRChat, It’s build up of 3d assets and all was going well and I’ve uploaded the world to the game. However I’ve come back to it to expand and now when trying to build I get this - https://gyazo.com/d465b79edde3b145c1f91dd6d2ea0042

My problem is that it’s probably fixable in C# but I’m not familiar with it and would love some help.

Thanks,
Andy,

anyone? Please.