Take a look at the sentence again:
Larissa will either
add a flock of plastic pink flamingos,
or she will purchase
a family of ceramic gnomes for the front lawn.
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 sentence, remove the comma after flamingos and change she will purchase to purchase.