Mesh error problem - General Questions - PrePoMax

I made U-Type shape aluminium material stamping nonlinear analysis. Before finish analysis, mesh has error like below figure. I don’t know how to solve it. If possible ,could you give me advise ? -----Material Setting---------- Density 2.7E-09 Young modulus 69000MPa Poisson’s ratio 0.33 Yield sterss Plastic strain 183 0 2450 0.2 —Surface behavior---- Type Linear K 30000 N/mm2 Co Deault — My code (Condition part only ) – PW : JfaBSxW7u

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Mesh error problem - General Questions - PrePoMax

PrePoMax & CalculiX - Basic Meshing

What exactly are we checking when we do buckling analysis? How to