I've not met one of these, but a few thoughts based on various forms of bus ticket machine -
Is it just one handle to operate, or is there one button / lever you have to press / hold in first to allow the operating handle to turn? (many bus ticket machines had something like this, to protect the conductor from issuing a ticket accidentally.)
As Dr Hoo has said, there may be a mechanical interlock so as well as the suggestion about the 'slug', are all the sliders properly clicked in to place? (and there may be a minor mechanical defect that means they look in place but don't line up properly any more, so go by feel rather than how it looks) and is the cover closed and lock / catch engaged properly?
And in your photo it's showing a zero fare value - have you tried with other fare values? Some bus ticket machines were set up so a conductor couldn't issue a zero value ticket, although this was more usually an interlock to stop you setting a zero value in the first place, rather than one that would let you set it up on the dial but not issue a ticket (while there were some exceptions, on a lot of companies there was no legitimate reason to issue a zero value ticket.)
does it feel like it's trying to move, or completely solid? It may simply be that the mechanism is very stiff, but I'd not recommend just heaving and hoping for the best - sometimes that can free a mechanism that's stuck, but it can also cause more damage.