I've tried many times to set up Goodreads in KOreader (the secret is SO long!) but am sure I typed in the whole thing correctly. The only way to test it (as it doesn't give you a direct response) is to my knowledge to search something. Every search has failed even though I checked and double checked the key and secret.

How do I find out what is going wrong?
I'm afraid "Goodreads" is a service almost nobody uses in KO, but my guess is only based in its wiki page: https://github.com/koreader/koreader/wiki/Goodreads and the fact that nobody raised issues in the last 4 years

About debugging: your best bet is to use the emulator on linux, mac or WSL and see if it works there. Whatever your issue is it is probably not related to the device that's running KOReader.

BTW, please update your device list or, at least, say your device platform or model when you ask questions. That could help getting more appropiate answers
The least error prone way of handling gnarly secrets is to fake it once in the UI, exit KOReader, then tweak the config file manually.
I mean, you must have entered the keys wrong. I just tested it on my end and it works. You should follow NiLuJe's suggestion and edit the settings/goodreads_*.lua file in KOReader's folder.

I'm surprised I don't see a way to mark the book as read there though... I tested whole plugin and it works as expected.

EDIT: Added some more information https://github.com/koreader/koreader/wiki/Goodreads

Oh, I actually would have thought KOreader synced read status to Goodreads or that I could mark it manually. Anyway - thanks for the link I will do that later today, will be SO much easier than to input it manually