Cannot enable Auto-Build in build target configuration

Symptoms:

• When I enable Auto-build in my build target configuration, it resets immediately after.
• I am not able to enable Auto-builds in my build target configuration.
• I had a build target configuration set to Auto-build but it has become disabled and I cannot re-enable it.

Cause:

• When a build target hasn't been built for 30 days, the Auto-build option is disabled automatically.

Resolution:

• To re-enable the Auto-build setting in the Configuration, the build target needs to be built manually once. Run a manual build of the build target (next step), then you should be able to re-enable Auto-build for that build target.
  
• To run a manual build, navigate to Build Automation > Build History, select Build, and then choose your target and build in the UI that comes up.
  

More information:

Build Target Configuration