Which One Country pass you're buying? Could you provide some screenshots of the problem? Please remove any personal details.
Perhaps it's the shipping time needed, you might want to buy a mobile pass or buy the paper pass at the closest international ticket office for you.
Paper passes have a long lead time as they need to be posted. They will not allow you buy a paper pass starting less than a month in advance.
Your options are to get a mobile pass or find somewhere to sell a paper pass in person.
Other members here have had passes issued in major rail stations in certain counties, I am not sure if RENFE in Spain sell them any longer though.
Depending on what country you wish to visit it may be possible to buy on arrival.
It is exactly 1 month onward so it has to do with often very, very long time for the post/correos.
IF you can tell us for where and how you go there, we may tell how it can be solved
DO note-this is a very big misunderstanding often, that 1 country is what it says; ONLY in that pais. NOT for the trip home to there. Buenes viajes.