Error: Translation set not known in …

Hello all. I searched around on this issue but never found a closure.
I have this plugin, but it seems to not get all of the strings to translate. It says it is 100% translated but I can’t translate login etc. Looking at the full file (? the file without a flag) with the translated languages, I see that file has more words than the translated files.
Not sure if I should do advanced configuration ? Or can I edit manually in .POT file?

I hope the description of my issue is clear enough.
Kind regards.