The valve is part of the cassette, not the toilet. If it is any way not closed fully, the cassette will not come out. The links Old Crow posted may help.
If you have ever opened and closed the valve manually when cleaning the cassette you might remember the yellow knob you turned to move the valve. If this is not lined up with the long dimension of the cassette, then the part of the toilet that actuates this is engaged with the yellow knob at some angle, which will prevent removal. Either by using the flush handle on the toilet or by physically moving the black valve (like described in the links) are the only two ways I can imagine accomplishing proper alignment of the knob so it will allow the cassette to slide out.