It's possible that the memory in your computer was fragmented or in use by other programs, so as a first attempt, restart your computer. Then start only Workbench, and open your Project. Right click on the system you want to solve and select Update. This will start the solver without opening Mechanical.

If you get the same error, then your computer does not have enough memory to solve that model using the Distributed Sparse Direct Solver.  The next thing you can try is to use the Iterative PCG solver which uses less RAM than the Direct Sparse solver.  In Mechanical, under the Analysis Settings, for the line that says Solver Type, select the Iterative solver instead of Program Controlled.  Click Solve and see what happens. Note that some models contain items that are not supported by the Iterative solver and those models will automatically switch to the Direct Sparse solver, so you will get no relief in that case.

If you must solve using the Direct Sparse solver, then you must reduce the model size. You can reduce the size by meshing with fewer, larger elements or changing the Element Order in the Mesh Details from Program Controlled to Linear. Note that if your mesh consists of Tetrahedral elements, Linear elements are not recommended.

The best long-term strategy is to install more RAM in your computer or purchase a new computer with more RAM.