Hello, I’m not sure to which video you are referring here?
Staking is not necessarily from the 1st of the month to the 1st of the next month. You can stake any time, but you will only get Ontology foundation rewards for stakes that were active between the 1st of the month to the 1st of the next month. Your stake will not be instant though, it will only count when the next round starts. You can check https://ontstake.o3.app to see how many blocks until the next round.
To get back to your issue, why will it not accept? Is it in a pending state? In that case all you need to do is wait until this round finishes, as you cannot start staking immediately.