Why is おかんじょうをください incorrect?

So I have learned that one of the ways to ask for the bill is おかんじょうをおねがいします。 I have also been told that you cannot say おかんじょうをください – why?

 

My understanding is that:

* おねがいします can be used for
* requesting an item – みずおねがいします
* requesting a service (e.g. taxi) – しぶやえきまでおねがいします

* ください can be used for

* requesting an item – みずをください
* asking someone to perform an action – ちょっとまってください

If I can say both みずをください and みずおねがいします to ask for water why can I not say おかんじょうをください to ask for the bill? Is this considered a service rather than an item?

Apologies if I’m missing something obvious.