If you encounter the error message Billing agreement id or transaction id is not valid with the error code [11451], it is due to a payment issue while trying to pay for their subscription using PayPal.
This issue happens when PayPal declines the billing agreement creation upon the purchase attempt. And because of this, the error is not being shown to the user on PayPal's end directly.
An example error message can be found below:
PayPal: Error creating transaction for customerId[] paymentToken[] transactionAmount[] error msg[Billing Agreement Id or transaction Id is not valid] error code[11451]
In order to fix it, you will need to contact PayPal's support to enable Reference transactions.
After this process is completed, new subscriptions via PayPal will be charged correctly, but existing PayPal subscriptions can not be renewed.
For existing users, the workaround (after Reference transactions are enabled) would be to cancel their existing subscriptions, remove the saved PayPal account, and subscribe again. The same PayPal account can be used to re-subscribe, it just needs to be authorized again.
After this is done, the new subscription will be renewed successfully when the time comes.