Split placement is where the linker takes (for instance) the various .text sections from the input files, and rather than combine them into one monolithic output section for placement, spreads it across two or more memories. Splitting is not allowed for certain sections like .cinit, .stack, and .sysmem, which must be contiguous. Other sections may be unsplittable depending on the target. What target ISA are you using?
The other reason a section might be unsplittable is quite a bit more cryptic. The simplified version is that it is probably in a GROUP that must be placed late, but split objects are not allowed to be placed late. You will probably need to move .myLocalMemory out of the group it is in, or change >> to > to disable splitting.