This message can sometimes appear for very large, difficult LP/MIP problems. It means that the Branch & Bound tree has exhausted all available main memory. Your best immediate course of action is to set the Task Pane Engine tab MIP group Node File Start option to a low value such as 1 (for 1GB of memory), or even a fractional value. This will cause the Gurobi Solver to write the Branch & Bound tree to a disk file (the “Node File”) once it reaches 1GB in size. This should allow you to solve the problem, at some cost in extra solution time. But it may be a good idea to upgrade your PC with more RAM to solve such problems.