Just a little note:
Enkel can mean grandchild as well as grandson, while Enkelin only means granddaughter. To specify that it is male, one can say Enkelsohn, or to specify that it may be of either gender, you can say Enkelkind (which may imply that he/she is still in childhood).
German kinship terms are similar to English ones -- paternal and maternal grandparents are not distinguished, nor are father's and mother's siblings distinguished from each other or from their spouses, but only by gender. (Onkel = "uncle" = father's brother / mother's brother / father's sister's husband / mother's sister's husband, and similarly for Tante = "aunt".)
The main distinction is that German distinguishes male from female cousins, while English does not. (But neither language distinguishes parallel cousins from cross cousins, or paternal from maternal cousins.)
I assume you're already familiar with why you say,
- Ich esse
einenApfel. = I am eating an apple.
EinApfel isst mich. = An apple is eating me.
It's the same idea here. "Enkel" is the object of the sentence, so the article takes an accusative ending to show that.
(The fun thing with being able to tell who or what is the subject versus object of the sentence with just article endings is that you can play with the word order for emphasis and still know what the sentence means. E.g. "Einen Apfel esse ich" still means "I am eating an apple" and "Mich isst ein Apfel" still means "An apple is eating me" . . . just with different words emphasized.)
What's wrong with the grandmother has a grandchild?
Nothing. "The grandmother has a grandchild." is one of the accepted translations.
There are no "suggestions" on Duolingo.
If you mean the hints that appear when you select a word: those are just that - hints. To jog your memory, but not to "recommend" or "suggest" anything.