Essentially, the approach doesn't meet the technical requirements to be considered a 'straight in' approach. Therefore even though you are basically aligned with the runway you can not use 'straight in' minima.
You are technically performing a circling approach (even though only a tiny correction is made in order to be fully aligned with the runway centreline). Therefore you need to use circling minima rather than 'straight in' minima.
The problem is the highly confusing wording that Jeppesen have chosen to use.
If you look carefully you can see that the statement 'Not Authorised' is written in the minima section of the plate. What they are trying to say is that there are no straight in minima authorised.
My company use LIDO charts which in this particular instance are much clearer. Instead of saying 'Straight in approach Not Authorised' they simply state: RW21 - Use Circling Minima.