Stricter DRCs were introduced in the 2013.2 release to prevent users from entering addresses that did not align with the addressing for PS7.
One of the DRCs was to ensure the addresses assigned were valid for the OCM.
In this case, the Error Message is not providing enough information to correct the issue with the assignment.
To work around this issue, you will need to do the following:
b) Manually assign the bottom LOWOCM space. (Drag it over the the mb Data)
c) Change its range and offset ( make sure you are choosing the correct offset)
d) Manually assign the second LOWOCM, (it will get assigned to its fixed
address of 0x00000000[1G])
e) Set its range.
f) Auto assign everything else.
Improvements have been made to the messaging in this situation starting in Vivado 2013.3.