I posted about this exact same error and my post was immediately removed as SPAM. I believe the problem is with the Paypal Sandbox. It seems to be acting up again today. Code that worked fine yesterday is suddenly returning error 400 today. No idea why the moderators are deleting posts about the problem.
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Worked fine yesterday and no code has changed. I come in this morning and the exact same code on the exact same page is giving the following error message: Error: Error: Request to post https://www.sandbox.paypal.com/v1/oauth2/token failed with 400 error.
{
"error": "invalid_client",
"error_description": "Client credentials are missing"
} The client credentials are definitely not missing, worked fine yesterday, and are still valid.
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Exact same problem. Using Sandbox seller account and Sandbox buyer account and Paypal gives me the exact same error message. There is very good reason Paypal had their lunch eaten by every one of their competitors. The documentation is always wrong. Sandbox accounts never work. The API is a mess. The BEST advice a developer can give a customer is to avoid using Paypal.
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