I have got an old version of openWRT (I´m using only for a test in an old router, I can not install a newer version, so I need to continue with this one).
you need to install luci-app-ddns. I believe that was available back in the LEDE days.
These errors are related to the fact you don't have a valid configuration (yet). when it completes installation, it launches the daemon which cannot do anything because there is no active and valid config, thus the error. Okay to disregard.
Please also understand that LEDE is not suitable for exposure to the internet anymore... it has many security vulnerabilities and has been EOL and unsupported for many years now. (I do understand your comment about the hardware situatoin -- but don't use that on the internet -- get newer hardware ASAP).
When you install the ddns scripts, it comes with a default configuration that has the entries disabled (because they're just examples, not valid real configs). Then, when the install finishes, it actually tries to run the ddns script, and it fails because nothing is configured+enabled. That is why you have those errors.
You need to edit the ddns configuration to put in your own valid config, then launch it again (or just reboot the router and it will auto-launch).
