The problem is that to reference a field in a repeat group you need to specify which iteration the application should consider. At the moment your instance name references the variable apeynom, which gives you an error.
E.g. you have two people called:
1 Ana Algo
2 Francisco Algomas
Which name should the instance name use in the concatenation?
The way to do this is to use the indexed-repeat
If you wanted to always reference the first name in the roster I suggest you create a calculated variable outside the loop using the indexed-repeat function; this new variable could then be included in the instance name.
Alternatively, a quick fix should be to remove the ${apeynom} from the instance name.
I hope this helps!
Best,
Andrea