This might be part of it seeing as you just joined the forums you will be a low trust level and it says cannot edit posts older than 24hr for the lowest level. This is also all configurable so it could change from default idk.
Your thread starter is 24hrs old now.
i just looked at my profile i am trust level 2 which by default i can edit my posts up to 30 days old. That explains it then because the thread i tried editing is way older than 20 days.