Summary:

Gridded Item Merge error: Message not found (IMMERGEDIMSMISMATCH)

Problem:

The gridded items being merged to not have the same grid dimension tags. 

Solution:

Update the item so that the grid dimension tags are the same.  Per online help: If the items are gridded, both items must have the same Grid dimension tags (e.g., Color/Size or Color/Size/Width), although the dimension values (e.g., Red, Blue, Yellow) may vary between items). For steps, see KB201.