Review the original item:
Sighing heavily, Melody tried to summon the energy to get to her next
class her heavy book bag and the long walk,
however, seemed impossible challenges.
You wanted to fix it this way:
Sighing heavily, Melody tried to summon the energy to get to her next
class,
her heavy book bag and the long walk, however, seemed impossible
challenges.
To add a comma between class and her would create an equally bad problem, a comma splice. A comma splice occurs when you have two main clauses joined with a comma alone. Melody tried to summon the energy to get to her next class is the first main clause. Her heavy book bag and the long walk, however, seemed impossible challenges is the second main clause.
The spot between class and her needs a stronger break than a wimpy comma.
You might want to consult the rules for fixing comma splices and fused sentences.