AJA_ and
wrp96 have explained it well. You are trying to book an open jaw where the outbound ends at one point and the inbound starts from another point. The outbound is the upper jaw and the inbound is the lower jaw, the surface distance between the outbound end point and the inbound start point is the open jaw. The open jaw distance must be less than the outbound distance and also less than the inbound distance. From the companion voucher guide thread:
Q: Can I book an open-jaw trip using the voucher?
A: You can, but you will need to call BA. For the trip to be valid, the length of the surface sector must be shorter than either of the flown journeys. If you want to fly from LHR to AAA and from BBB back to LHR, then the LHR-AAA and BBB-LHR distances must both be greater than the distance between AAA and BBB.
https://www.flyertalk.com/forum/brit...er-thread.html
While I'm not entirely sure I know what you are trying to do but it seems to me you are trying to book LHR-DUB, MIA-LHR. This is not permissible as the distance of the open jaw -- DUB-MIA -- is greater than one of your jaws, in this case, your outbound LHR-DUB.