How do I get a job in SAP MM? I am a design engineer (mechanical field), and I want to change my field.

You may have that as a part of design engineer when you worked for a manufacturing company.

If you have experience and expertise in materials management, SAP MM would make sense. You may have that as a part of design engineer when you worked for a manufacturing company.

Without that functional knowledge in materials management, learning SAP MM will not help you much.