Can I make grapes honey from grape juice only

This type of food is known in English as a "treacle", and it can be made from a variety of fruit, not just grapes.

You can surely make it from pure grape juice. You don't need to add sugar, the whole point of treacle is to concentrate the sugar of the grapes to use it as a sweetener on its own. Adding sugar would be redundant.

You don't need to make it with whole grapes, in fact it is better to use juice only. The fewer dry mass you have, the less burnt off-tastes you will get. Also, some parts of the fruit will never dissolve fully, as Stephie mentioned, so with whole fruit, you normally have to sieve it. It is easier to start with the juice already, then you only have to boil out the major part of the liquid. The reasons that it is traditionally done with whole fruit is that 1) it is less effort to boil, then sieve, than to press juice (sieving all the while) then boil, and 2) you will get more treacle per kg of grapes if you start with the whole fruit (although it will be less refined).