I want to create a client and he already exists in the list of my user table.

I want to create a client and he does already exist in my users table, how can I match ?

You must click on the "Clients" tab, then on "New". You have to check the box "Assign a Joomla Account" and then you will see a select dropdown with the list of your users. Just select the user you want. If you have a lot of users, you can push on the keyboards corresponding to the first letter and it will show you all users beggining from that letter. For example, if I look for "Dupont", i must click on the arrow to detail the dropdown, then i pushed the "D" key and i select "Dupont" on the list by clicking on it.