Hello kisa, does this happen also with other projects or just this one? in case it's only this one, could you send it to contact@esotericsoftware.com so we can help you figure out what is preventing you from exporting?
We also recommend to use beta to export gifs or videos.
In beta you can also set the number of loops for video, the size (maybe your skeleton is very big and your computer doesn't have the requirements to export it) and other settings.
You can check the output of the export in beta by clicking the Preview button in the upper corner of the export window.