Sandbox PayPal Checkout Popup not open

[piyushjpatel]

Error: Request to post https://www.sandbox.paypal.com/v1/oauth2/token failed with 400 error. Correlation id: 236a888cbe528, 236a888cbe528 { "error": "invalid_client", "error_description": "Client credentials are missing" } request/</<@https://www.paypalobjects.com/api/checkout.js:14063:39 ./node_modules/post-robot/src/index.js/_RECEIVE_MESSAGE_TYPE[conf.b.POST_MESSAGE_TYPE.RESPONSE]@http://www.paypalobjects.com/api/checkout.js:2569:31 receiveMessage@http://www.paypalobjects.com/api/checkout.js:2611:60 messageListener@http://www.paypalobjects.com/api/checkout.js:2632:13 ./node_modules/post-robot/src/index.js/_RECEIVE_MESSAGE_TYPE[conf.b.POST_MESSAGE_TYPE.RESPONSE]@https://www.paypalobjects.com/api/checkout.js:2569:31 receiveMessage@https://www.paypalobjects.com/api/checkout.js:2611:60 messageListener@https://www.paypalobjects.com/api/checkout.js:2632:13 serialize.js:175:14

How to resolve this issue?