Description
If you have tried assignment pin location in Pin Planner before you do any full compilation, the pin planner will try to create mem_ck[0](n) as differential pair for mem_ck[0].
To re-assign mem_ck_n[0] as differential pair for mem_ck[0], you could try adding below assignment in Assignement editor:
set_instance_assignment -name CKN_CK_PAIR ON -from mem_ck[0] -to mem_ck_n[0]