Here is the item:
Josh neglected not only
to thank Solita, but
he also
gave a sip of orange soda to Reva, Solita's rival.
When you use not only ... but also, you must have two equal grammatical items. After not only, notice that you have an infinitive phrase, to thank Solita. But after but also, you have a main clause, he gave a sip of orange soda to Reva. The structure is not parallel.
You made a good choice, grammar gourmet!
To fix the problem, you should start the item with Not only did Josh neglect to thank Solita.