Sorry to hear of the problem you are having.
Please make sure you have administrator role on your site or create new user having this role.
This can be due to plugin conflict on your site so please try temporary deactivating all plugins except Unyson plugin and see whether everything works fine and then enable the plugins one by one to see which plugin is conflicting if any.
Are you using nginx?
Sometime the nginx configuration not being compatible with the latest version of WordPress so please contact your host.