I know this was weeks ago and you've probably figured this out by now but...
I'm trying to get Geoblocker working on my Nextcloud today and got this same error. I got rid of it by installing the Geoblocker required packages:
sudo apt-get install geoip-bin geoip-database
And to check if exec is allowed in PHP, open php.ini and search for 'disable_functions'. If exec isn't in that list, reasonably sure it's enabled.
I can't say I've gotten Geoblocker working yet because I'm using Nginx as a reverse proxy to a Apache2 web server, and my external ip keeps showing up as a localhost lan ip. If I can get that corrected with the external ip I think I'm there. Something something 'HTTP_X_FORWARDED_FOR'.
Good luck.