How to rebuild poly-surface?

スクリーンショット 2021-01-22 094447

I want to rebuild a selected item.
But I cannot rebuild it.

Please advise.

Hello - Rebuild works on single surfaces - that appears to be a polysurface - the What command will tell you.

What is the difference between a single surface and a polysurface?
Please tell me how to convert them.

The best way would be to rebuild it manually, like, dup the edges of the selected poly surface, rebuild the edges courves to be of a similar, ideally, same degree, and then do an edgesrf or sweep2 command.

the difference between a single surface and a polysurface is on the name, a single surface is itself the whole surface, while the polysurface is made of multiple surfaces.

Hello - please make use of the Help in Rhino - it is quite complete. See also -