The solver estimates how much RAM is required to run in-core.
Did the solver say it started in-core or out-of-core? If is says it started out-of-core, you can force it to start in-core by using a command object with this code:
Note that there are two commas. This assumes you are using Distributed ANSYS and checked that box.
Even though you have more physical RAM than the estimate, it may switch from in-core to out-of-core during the solution because more RAM was required during the computation than was estimated at the beginning.
You can have the best chance of success if you Restart the computer and only run the solver and no other software to maximize the size of a free block of RAM.
If you really want it to run in-core, reduce the node count.