Paypal security header is not valid error message can be shown because of the following reasons:
- 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
- You have white spaces in your credentials. Eq. api_username_123gjh123g – 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 QUICKAD or QUICKJOB CMS Paypal fields.