I am absolute new to OSM and all the tools and file formats around it. I checked around in the forum and found a link to this tutorial which “draws” (?) ZIPCODE (“Postleitzahl”) areas of Germany and label them.
2.18 is just “stable” in Debian. But I see there is a 3.10 in the backports repository. I will check that.
I do know know anything about a connection between QGIS and OSM. I imported some files as described in the tutorials. I am nearly sure that the field names are correct and comes from that imported files.
But the question is not what cause the error. The question is how can I see the concrete and full error message? Currently I only see code in red characters. I need more details. Is there a debug console or something like that in QGIS?
I’m sorry, but investigating the actual source of the data you link to requires far more time than I can spend. The shape file may stem from OSM, but it has clearly been manipulated because the field names do not match OSM native values. Furthermore if it comes from OSM it does not contain a mandatory attribution statement to comply with the licence.
The current LTR for QGIS is 3.16 (I would never rely on the Debian repo for any GIS related stuff (OSM specific software is probably worse), but again I can’t spend time supporting software which really is not even remotely current. I tend to run quite old versions of QGIS because it is very stable, but even I upgraded to 3.x in 2018. The problem with 2.x versions is that there were fundamental changes in core 2.x=>3.x and consequently a lot of plugins changed considerably, or stopped being supported. (I suspect migration of Python versions also occurred at that time).
However, I think what you need is more basic QGIS support. Please check the actual field names in the files you have downloaded, they may not correspond to those shown on screen in the tutorial.