There are some power apps and some Nintex workflow version ( OnPremise, 0365 and Cloud ). Every product have many ways to connect/combine. What's your specific need?

We use office 365 and share-point online

Is there something specific you are trying to accomplish with PowerApps and Nintex? They absolutely can work together. Are you using PowerApps to create a Form? You can't use Nintex Forms with PowerApps because they are separate. It's one or the other. But If you wanted a PowerApps form and a Nintex Workflow you could do that. Or if you want a Nintex Form and a Flow workflow, you can do that. Flow would probably work with Nintex Workflow. Just an assumption because I haven't actually done it, but you could design a Flow workflow and have a Nintex workflow run at the same time or even after the Flow if you have Flow update some field and tell Nintex to not start until that field is updated.

Thank you so much for your reply.

Hi everyone, my question is how can I make the Nintex WF to use a form created with PowerApps?

So far the Nintex WF wants to make me create a Nintex Form.

You can't first create a PowerApp form on a list then create a nintex WF? Does it give oyu some kind of error? What is it showing?