Security Header Not Valid When Setting up Paypal

Paypal security header is not valid error message can be shown because of the following reasons:

1. You have set up 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
   
   
2. You have white spaces in your credentials.
   Eq.              api_username_123gjh123g – You see how much whitespaces after the Eq.
   
   
3. You put wrong API credential in wrong place ex. API Signature data you put in the API Username field inside QUICKAD or QUICKJOB  or QUICKVCARD  or QUICKQR or QUICKHOMES CMS Paypal fields.