I guess that you are trying to make reservations at the Interrail/Eurail website. The advice from the experienced travellers in the community is to use other ways to make reservations than the Interrail/Eurail website. You can look at the guide in the link:

https://community.eurail.com/train-connections-reservations-47/how-to-get-reservations-105

If you, after having looked at the guide, have questions about how to make specific reservation, please create a new topic with your travel details (departure date, time and route) and you will get advice.

There might be a a whole host of reasons as to why you cannot proceed with paying with reservations. A few things to keep in mind: 

• Paper tickets: if you are traveling soon, it might not be possible to proceed to payment as we cannot have tickets posted to your location in time. Simply remove the reservations issued as paper tickets and retry. You can always book these locally at the station. 
• The trips/travelers are not selected. 
• Personal browser issues. Clear your cache and cookies and try again or try reserving from a different browser. 
• You are making too many reservations at once. You can only make reservations for up to 9 travelers at a time. You can also split your reservations up in 2. 

If the issue remains, there might be a technical glitch at play. Please reach out to our customer service team so that we can work on a solution asap :)