When you try to add a subdivision surface modifier you may encounter a problem where it won't subdivide past 0 in viewport. When you also try to directly apply the modifier you get an error message that says "Modifer is disabled, skipping apply". What causes this issue?
This may happen if you have Cycles enabled in Render Properties and "Simplify" checked where it determines the max subdivisions users may enable for the mesh. This screenshot shows that the max subdivision has been set to 0 and therefore restricts the modifier from applying any subdivision to the mesh.