I recently built a 32-bit Windows desktop app. I immediately wanted to build a 64-bit version, but the Build button is greyed out. I can open other projects and it is not greyed out. I have restarted Windows and even opened the project on a different computer after copying the source file to the other computer. Selecting 32-bit again does not fix it either. I am running Xojo 2019r1.1 on Windows 10 (1903) and have a current Pro license installed on both machines.
Make sure you dont have any other targets selected. For instance it would be grayed out if you have macOS selected.
Somehow I unchecked the Windows target. That fixed the problem. Thanks Greg for Pointing out where to look.