Fast-forward merge is not possible. Rebase the source branch onto the target branch or merge target branch into source branch to allow this merge request to be merged.
:javascript
$('.rebase-mr-form').on('ajax:send',function(){
$('.rebase-mr-form :input').disable();
});
$('.js-rebase-button').on('click',function(){
$('.js-rebase-button').html("<i class='fa fa-spinner fa-spin'></i> Rebase in progress");