What the ****

[Serpius]

What's your MyBB version?

Did you manually set a PIN at all?
Check ./inc/config.php

If there is your PIN defined, did you have made any other changes to files or PHP code?

[ExiTuS]

1.8.21

Yes I manually set the PIN, because I used softaculous

And no I did not changes

Using Softaculous to install a new MyBB website is a mistake.

You should have used the 'manual installation' method.
You will have fewer problems if you do a manual installation.

Back to your issue, the PIN is not being properly set during the Softaculous installation. 
For some reason, Softaculous does not do a good job of this.

You have to manually edit the config.php file.