XF 2.1 This installation is in development mode. Upgrades must be installed manually.

[AndyB]

Why is it not possible to upgrade while in development mode?

 

[DragonByte Tech]

Because of the _output directory. When development mode is enabled, all installations (add-ons, XF, etc) expects to read from an _output directory, which naturally does not exist when using the 1-click upgrade method.