Ensure your hardware complies with the System Requirements for Inventor.
https://knowledge.autodesk.com/support/inventor-products/troubleshooting/caas/sfdcarticles/sfdcarticles/System-requirements-for-Autodesk-Inventor-Products.html
Ensure that the latest Service Packs are installed.
For Inventor;
https://knowledge.autodesk.com/support/inventor-products/downloads
For MDTools;
https://VESTusa.com/Manifold-Design_MDTools-700-Series_Service-Packs.htm
Ensure that when you begin the process of designing the block in MDTools you are in the Virtual Cavity Mode as this uses less memory. Convert the manifold to the Feature based mode only after the design is complete and ready to create the drawing.
For larger manifolds, refer:
https://VESTusa.com/Download/Inventor-MDTools-Settings-for-Optimum-Performance-with-Large-Manifolds.pdf.