Why do I get an error whenever I publish a plane on ROBLOX?

Kia Ora,

I have built a plane on ROBLOX however when I go to upload, I see the following message:

'Error: Upload Failed. Please try publishing again.'

Image: https://gyazo.com/f12faa9899aa4aaedc5fa49ea59d5e96

I have been through and deleted some un-necessary unions but still receive the message.

Any help would be greatly appreciated :)