To tweet from Nelio Content you have to authorize Nelio Content to access your X account. Go to the plugin’s Settings » Social Profiles tab and click on the X icon:

A dialog box will appear in which you will have to select whether you want to connect X directly through Nelio Content or you want to connect your X profiles through Buffer.
Keep in mind that currently the social network X takes into account the number of messages that can be published through its API in short periods of time. For this reason you may be interested in connecting through Buffer.

Connect Your X Profiles directly with Nelio Content
Click on the X icon and a new window pops up and asks you to authorize Nelio Content to use your X profile.

If you didn’t have your X session open, X will ask you to log in first in order to grant Nelio Content access:

In both cases, after clicking on Authorize App, your list of Connected Profiles will end up including your X profile:

Connect your X profiles through Buffer
Alternatively, you have the option to connect your X profiles through Buffer as explained in the following link: