FutureBasic 5, a 32-bit app, requires macOS 10.7 to 10.14 but won't run in macOS 10.15+.
It is a good option for those unable to use the latest FutureBasic.
The link below details FutureBasic 5's installation options.
FutureBasic 5 installation notes
Apple began removing specific key header files in macOS 10.7 which were needed by FutureBasic 5.
FixSDK solves the issue by restoring the missing files.
Later FB releases don't require those removed headers, so today FixSDK is unnecessary.
See this page for more details on the overall issue.
All SDKs at the links below, except the one for OSX 10.6, are "fixed".
The FixSDK app automates the fixing of older SDKs. It can also be used to backup and restore SDKs between Xcode releases.
A selection of demos that run in FB5.