And the website doesn't even tell me why. Additionally the analysis option is greyed out.
If analysis is greyed out, most likely the position you set up is illegal e.g. one player doesn't have a king or a king is in check even though it's the opponent's move.
It will also be greyed out if the variant is anything other than "standard" as for now, games "from position" in variants are not possible.
That only applies to "continue from here" though. If "analysis" is also greyed out, it generally means the position is illegal.
