At the ..... level/ on the ..... level

It depends on the context; if the context is one of possible administrative support at different levels, then to make that clear in your enunciation you use "at". On the other hand, if the context is not one that includes that notion you use "on".

An examination of the usage for "at" and "on" from the following ngrams will show, however, that it is difficult to make the difference (at, on) . This can be seen also from this next comparison: at the national level, on the national level.