Hi @vlad.lyt and welcome to the Mattermost forums!
Can you confirm that all the relevant parts of the permissions have been set accordingly?
If I disable the ability for members to create teams, the button will disappear for them:
You’re in the “Team Scheme” which is only relevant for the team you’re currently in. You need to go to “User Management” → “Permissions” and change the permissions for “All members” in there.
I would like to express my sincerest gratitude for your product and for resolving the issue we had. It was a pleasure working with you and we appreciate the level of professionalism and support you provided. Once again, thank you very much for your assistance.