What tyres will fit this ebike rim? Out of an impulse buy - I now have a complete ebike kit with battery.
Originally I wanted this to be put on a mountain bike but upon seeing how little the rim is - I don't think any mountain bike tyres will fit in there.
I measured the rim (where the tyre will go) and it was roughly 2cm, slightly less in fact, does that mean that this kit is designed to go on a road bike/hybrid bike and only that?
The diameter is 26 inches - any help appreciated thanks
Presumably you have purchased an eBike kit with a in-wheel motor, so you have a wheel but you don't know what size the rim is. What did the kit specifications say about the wheel? Is there a marking on the rim?
An approximately 20mm wide rim is wide enough for mountain bike tires up to 60mm / 2.25 inches in width.
The bicycle wheel diameters in common use are:

ISO/ETRTO 622 - 622mm rim, known as '700c' on road bike, '29 inch' on MTBs
ISO/ETRTO 584 - 584mm rim, known as '650b' on road bike, '27.5 inch' on MTBs
ISO/ETRTO 559 - 559mm rim, known as '26 inch' on MTBs

The ISO/ETRTO number is the diameter of where a cincher tire mounts on the rim. The MTB sizes refer to the nominal diameter of the whole wheel with a tire installed, not the rim diameter. You can  measure you wheel rim to figure out what you have. 26 inches is 660mm so I think that measurement is an error.
Bear in mind that bicycle frames are designed for a specific wheel rim diameter. You can't put a 622 wheel in a frame designed for a 559 wheel. You also need to pay attention to the hub width, wheel attachment (nuts, quick release etc) and rim vs disc brake when considering frame compatibility.