did you consider to reduce the mesh size? When I reduce it, the model meshes without a problem. The model is very thin, you should consider using a shell or beam element mesh.
As Janko mentioned, your mesh is too coarse. Reducing it to max/min element size of 5/1.6mm will mesh it, however these are weldment structures and across the thickness it is recommended to have at least 2-3 high quality elements, which following this results in considerably high amount of mesh elements that will tax you a lot on computational time. Having said that, using beam elements is the best practice for your analysis.