How to get fulltextsearch running?

My setup:
Nextcloud 18.0.4 (direct install, not Docker)
MariaDB 10.*
php 7.3.*

I tried a
root@nass666:/volume1/web/nextcloud# sudo -u root php ./occ fulltextsearch:index
Which returns:
This version of Nextcloud requires at least PHP 7.2<br/>You are currently running 5.6.11. Please update your PHP version.roo

Well, in Nextcloud System ist says: 7.3.11 - Anyway, I am not sure if that is related.

Current status is fulltextsearch is installed via apps and I get the search icon. I have a few readable PDF in the /Document/ folder. But when I search something from the search page nothing happens.

AIM: I have a lot of scanned and OCRd material that I like to search through without looking at each multipage PDF one by one. I do the OCR while scanning and have no need for the (I guess resource hungry) Nextcloud OCR.