I have no experience working in Abacus. I need to compare the solution of the code with the solution in Abaqus. How to make the C3D8 and C3D20 elements in the Abaqus?
You do not need to make these elements in Abaqus. The elements are present in default. However, it is better for you to understand the difference before comparing with the MATLAB.
If you want to make an element you need to code that specific element yourself.