Your system of equations is close to 5.5Mil dofs. At those numbers, direct solvers will get very memory intensive, which is probably the problem you’re running into. One option is to expand on the RAM your compute system has. But more appropriately, it is probably time you start to invest in getting an iterative solver to run. Those are way less memort intensive, and in general the way to go for solving big problems.
In a nutshell, you’ll want to use GMRES with appropriate preconditioning. The preconditioning is really the crux, and also the main difficulty. On the forum you’ll find multiple threads that could be of use. An old thread of mine comes to mind: Precondition appears not to happen - #5 by Stein