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 :-)