The 10002 Security error - Security header is not valid error message indicates that your API credentials aren't being accepted by PayPal.
1. You have your PayPal module configured to "Sandbox" but have not used Sandbox API credentials.
2. You have your PayPal module configured to "Live" but have not used Live API credentials.
3. You have mis-typed one or more API credentials entries, including but not limited to leaving trailing spaces.
4. You have deleted and/or recreated new API credentials in your PayPal account but have not copied those new credentials back to your store.