Hello everyone,
This is my first post.

My goal is to have a master domain www.mydomain.com and slaves sites who are independent domains, for example: www.domain1.com, www.domain2.com, www.domain3.com, etc...
So, I added a template and is the only field filled in DB prefix: {site_id}_
and of course I set the report folders.
Then I added a new site by putting in list domain: www.domain1.com, domain1.com
I entered the parameters for the db, and I quote the path to the deploy folder: I use ISPConfig 3, and then my root directory is /var/www/clients/clientX/webX/web

The error that gives me is:
JFolder:: create: Could not create directory
JFolder:: create: Could not create directory
JFolder:: create: Could not create directory
JFolder:: create: Could not create directory
Error writing configuration file [/var/www/clients/clientX/webX/configuration.php]

The web space is writable, and I think the path of root is correct.
Definitely something wrong, but I do not know what.
Thanks for the help.

I don't know the HTTP Server administration tool that you are using.
If you are using cPanel.

Go in cPanel AddOn domain and create the domain with a "document root" like [home]domains/xxx.com/public_html

When the domain is created, ensure that you have the correct path.
You can check it with a "hello world" that you put in the document root of this domain.
It will display the path that must be used in JMS "deploy folder".

Go in JMS and create a slave site
Site ID = xxx
Domain = www.XXX.com, xxx.com
Deployed directory = [home]domains/xxx.com/public_html (the full path corresponding to your domain).

You can get inspired of the tutorial video CP-04 that show a similar case with a subdomain. You just have to use the AddOn domain instead of subdomain.
www.jms2win.com/en/tutorial#jms12x-CP-04

You must use the AddOn domain and not another cPanel account to have the same ownership of the files and folder. This avoid permission error.

If you are trying deploy the slave site into another cPanel account, you should grant the permission of the account to allow writing in both account. It is easier to use a same account and the AddOn domain.