We are planning to upgrade Project Server 2010 to Project Server 2016.
We are using Nintex workflow for SharePoint 2010 and Project Server 2010 and we need to upgrade it also to 2016.
Is Nintex workflow for Project Server 2016 available or not? if not, can we use Nintex Workfow for Project Server 2013?
as far as I know there is no Workflow for Project Server 2016 and it's unsure when it will arrive. You won't be able to install Workflow for Project 2013 to the 2016 environment as the installer checks the appropriate installation of 2013 and won't find 2016 as correct installation.
I think you should get in touch with your Nintex Partner or directly with Nintex Support to ask them when there will be Nintex for Project 2016 to get a final answer.
I made some assumption based on my own experience as end user. After researching this a little bit more, I may have spoken too soon. Probably best to reach out to Nintex Support. I found a couple of help files for 2013, but nothing for 2016. Don't know if that means it's not available yet, or just don't have the help files.