In the branch manager inside the Desktop Modeler, you can enable a branch for Web Modeler collaboration. See Project > More versioning > Manage branch lines.
See also https://docs.mendix.com/refguide/collaborative-development for the official documentation.
Dear Arjan,
I created a branch from main line and enabled it for the web modeler,
and it worked! Thanks Arjan.
Best Regards,
Sameh