If you composed original songs by using Yamaha VOCALOID6 Voicebank, you can sell them without entering into licensing agreements or obtaining permission in principle, regardless of the purpose.

However, there might be some restrictions depending on voicebank.
Please refer to the EULA that is included with each voicebank.
Additionally, please contact the company responsible for third-party products and comply with Article 4 in the EULA.

In particular, please note the following items:
1) Disclosing or providing synthesized voices with lyrics that violating public order and morals.
2) Disclosing or providing synthesized voices with lyrics that violate not only singers (voice actors) in this product, but also honor, personalities, and other rights of third parties.