Easy error to fix, once you know what's going on (is not always the case?).
Just upgrade your system from php 5 to php7. Drupal8 requirement.
In BLT, just edit your box/config.yml and change this line
php_version: "5.6"
to this:
php_version: "7.2"
Good old vagrant provision should do the rest of the trick. You are welcome :-)