Hello @Kirill Volkov
Welcome to the community.
As per this article, you must be a member of the site-admin group to add apps to the JIRA instance.
Thanks @Trudy P Claspill I checked previously and I am sure that I definitely am as an admin
There are multiple levels of access for administrative functions based on the User Groups to which you belong. To which user groups do you belong? Do you specifically belong to one called "site-admins"?
Reference: https://support.atlassian.com/user-management/docs/create-and-update-groups/
I found where I was confused !Thanks a lot for helping @Trudy P Claspill .
I have to put here a screen which can helps other people figure out where the needed setting are.Here it is :
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