This can happen by few reasons
You have setup LIVE credentials and you have turned Test mode to ON. If you want to test you need to set TEST credentials and have Paypal Sandbox Account
You have white spaces in your credentials. Eq. api_username_1a242jdj2455 – You see how much whitespaces after the Eq.
You put wrong API credential in wrong place ex. API Signature data you put in the API Username field inside Paypal fields.