This is a known issue. In Planning mode, the Build button can stop responding. The team is aware and is tracking it.
A couple things you can try as a workaround:
Start a new chat and run the plan again. Does Build work in a fresh session?
After the plan finishes, try reloading the window (Cmd + Shift + P, then “Reload Window”), then click Build.
Also, can you reproduce this reliably, or did it happen just once? And could you copy the Request ID from that chat (top-right menu, then Copy Request ID)? That’ll help us dig in.