I can enter a prediction but it will sometimes not save. An error message like "The prediction is not valid" is being displayed.
Probably your prediction community has got a strategy prohibition.
Strategy prohibitions prevent players to predict more than 2/3 of home wins or too many equal results (2:1, 1:2).
Please take a look at the rules of the game inside of your prediction community to get to know the rules you have to follow.
Even though the match hasn't started yet, I am not able to enter a prediction. The input fields are missing.
The lead time has already expired.
The lead time can deviate from the kickoff time. Take a look at the game rules, how the administrator has configured the prediction centre. It happens that you have to predict all a matchday's matches before the first match of it.