Hi! Welcome to the forum!
You are likely using multiple email addresses and therefore different accounts. The account that you are currently using to post in this forum does have a subscription, and should work just fine when used!
Please ensure that you are signed into the correct account before authenticating in the Charts app. You can verify this by opening navigraph.com/account/subscription - if you see your subscription there then signing in to Charts should work correctly!
If this does not work, please contact account support at account@navigraph.com !
Kind Regards,
Malte