Here is the item:
Larissa will either
add a flock of plastic pink flamingos,
or she will purchase a family of ceramic
gnomes for the front lawn.
Nice choice! When you use either ... or, you must maintain parallel structure. Add, part of the verb phrase will add, follows either, but she will purchase ..., a whole main clause, comes after or, wrecking the parallelism.
To fix the item, remove the comma after flamingos and change she will purchase to purchase.