Why does a message with respect to the presser feet #1C and #1D appear when I start to sew?

This error message appears when:1) Wrong foot is attached. When working with the BERNINA Dual Feed, only 'D' feet may be used.2) BERNINA Dual Feed hasn't been properly deactivated. Refer to 'BERNINA Dual Feed' chapter in the user manual. In order to ensure functionality of the sewing computer it is essential to ALWAYS lower the presser foot before engaging or disengaging the BERNINA Dual Feed.