One reason is incorrect credentials, credentials are case sensitive and extra spaces are counted as a character.
Also if you are using IP Address Whitelisting in your PayPal Manager Account and the IP address of your Webhost is not listed as an allowed IP address, you will receive User Authentication Failed. Please log into your PayPal Manager account, go to Account Administration and click on Allowed IP Addresses to confirm that you do not have IP address Whitelisting enabled.
If you are certain that the credentials are correct and you do not have IP address Whitelisting enabled the other reason you can receive User Authentication Failed for recurring payments is that your sandbox account is not setup for recurring payments.
If you log into your PayPal Manager Account: https://manager.paypal.com there is a banner that shows on the home page to setup recurring payments. Ensure that you have subscribed recurring payments.
Once Recurring Payments is setup in an Account, under the Service Summary section in the bottom Right hand side of the Home Screen, it will show Recurring Billing.
Once you have confirmed that you are fully subscribed to recurring payments, you aren't blocking an IP address, and your credentials are correct you will no longer receive User Authenitcation Failed.