[Installing] Install stack at latest step

[StefanT]

You can always just remove

$output = $CAPI_Util->GetRandom($count, 0);

from inc/functions.php.
However I'm unsure if it's a bug in MyBB or somewhere else.

Enabling openssl module for PHP would solve the issue, too.

EDIT: Bug report: https://github.com/mybb/mybb/issues/750