This is expected behavior. After installing and activating a plugin, you are supposed to Rebuild & Restart. There's a special button in the ACP to perform this.
Then there is no problem. Thank you for your answers.
Today i installed
and now when i post , i have problem with database and freeze site.
Update 16: 18 : I removed nodebb-plugin-poll and Seems to me that returned all right.
For me is this.