Here is the sentence:
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. Good choice, grammar gourmet!
To fix the problem, you should start the sentence with Not only did Josh neglect to thank Solita.