The mixed order mesh elements error occurs when you have following condition: 1. A shared multibody part containing solids and surface bodes, and 2. In Mesh settings, under Defaults options, Element Order is set as “Program Controlled” Under this condition, solids get meshed with higher order element while surfaces get meshed with lower order. This results in mixed order meshing which is not supported by Mechanical solver. To overcome this issue, simply change the Element Order option to “Quadratic” which will mesh all solid and surface bodies with higher order elements. Refer attached image which shows this option.

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