Upgrade from Tourer to Tourer+ failed

We were unable to complete the resubscription because we could not charge the customer for the resubscription - Code:functions/failed-precondition

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I think it depends how you did the the purchase.
When you purchased over the website with paddle, you have to use paddle again - or when Playstore, then Playstore. Please let us know if it works.

This issue can happen if there is an issue with your existing payment method. Usually credit cards expire after some time.

If you used Paypal the subscription agreement between Paypal and Paddle are sometimes cancelled.

You need to update your payment details them at Paddle. You can use the update payment method link in your account here: https://kurviger.de/account/tourer/en