Required "client_secret" key is not supplied in config...

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This error message occurs most commonly on Magento 2 Cloud instances.

In each case so far we have found that the issue was due to the Magento Encryption Key not being properly set, preventing the sensitive config data from being saved correctly.

You can use this guide to set up the encryption key. Then, enter the client_secret key again in the Subscribe Pro extension configuration.

Make sure to flush the Magento cache after completing those steps, and then try to test the connection again.