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_ 1a242jdj2455 – You see how much white spaces after the Eq.
3. You put wrong API credential in wrong place ex. API Signature data you put in the API Username field inside Ultimate CRM Paypal fields.