If you are a returning player it could be that you dont have the latest client version. You need the Valentine21_v2 Edition which you can download here.
If you already have the latest client version it could be that you have to merge your Red & Blue accounts.
Check if you have the option to merge on the dashboard and make sure to select the right server as your main server.
https://dashboard.pokemonrevolution.net/dashboard/
If you are unable to login after merging your account, you will have to reset your password on the dashboard as well.
For more info about the merge and about the errors that might occur with it, take a look at the following link:
https://pokemonrevolution.net/forum/topic/90465-everything-about-the-merge-general-faq-issue-faq-read-before-making-a-merge-issue-thread/