The “Compression Only” support creates rigid-flexible contact for the boundary condition. As this support constrains the motion in normal to scoped faces only, there may be issues when there are inplane motion like sliding. The solver diagnostics try to ensure that the model has least one constraint for each DOF (UX, UY, and UZ for a 3D solid model) and at least 6 total constraints to prevent rigid body motion. However, the solver may not recognize that the model is adequately restrained and it throws warning messages about Weak Spring added into your model.