Can anyone tell if I have a Nintex license for SP 2016 on premise, can the same license be used for SP Online? I have an online tenant too and we are going to migrate sites from SP 2016 to Online version. Can the same license be used for both 2016 and online version?
if your license is based on a subscription, yes, you can use it also in your tenant environment (you should be able to add your tenant to your license, ask your partner how to do it)
if you're still using old perpetual licenses for on-premises, I'm sorry but SP Online is not covered and you need to buy/change your license..