Connection failure - error code : 10002, error message : Security header is not

-- url of the page with the problem -- : imbalicrafts.com
-- HikaShop version -- : 4.4.2
-- Joomla version -- : 3.9.26
-- PHP version -- : 7.4.19
-- Browser(s) name and version -- : Google chrome 90.0.4430.212

Hi,

I have recently set up my new website and am currently using hikashop starter (I plan to upgrade if I can get it to work). I have set up both express paypal and paypal plugin and entered API details. I get this error message when the express paypal plugin is published:

MESSAGE
Connection failure - error code : 10002 , error message : Security header is not valid

And when only the paypal plugin is published, the sale goes through, but the purchase is not confirmed by paypal and I manually have to set the sale from 'created' to 'completed' in the backend.

so in a nutshell:
- error 10002 from paypal if express plugin is published
- paypal does not 'confirm' purchase automatically I have to do it manually when paypal plugin only is published.
- how do I skip registration of address etc... please to go straight to paypal? I don't want any customer details on my website.

Sorry if this is all an easy fix, I have been out of the loop with Joomla for a good number of years so I am really not that clued up with things until I get a bit more time to familiarise myself with things again. I apologise in advance if this is just a few simple settings. I have googled first but couldn't come up with anything that helps me. Oh and it's a live site and not a sand box environment. Any help appreciated Thanks in advance!
Nadja

Thanks for the reply, much appreciated! I will go through all the documentation you have provided and hopefully will be able to sort everything out.

I can't figure out what I need to do with the express plugin. I don't know how to create an API that works in paypal. I thought the one I got was provided by paypal.
I have unpublished the express plugin and changed the workflow to remove customer login. Everything appears to be working but when a customer checks out to be redirected to paypal the error message I get is

'Things don't appear to be working at the moment. Please try again later.'

Hello,

This kind of error generally occurs when payment methods are not properly configured, can you show us screenshots of how you configured your:
- Payment plugin via "Hikashop->System->Payment plugins"
- Checkout workflow via "Hikashop->System->Configuration->Checkout workflow"

Kind regards,
Mohamed.

I managed to fix the 'Things don't appear to be working at the moment. Please try again later.'
error and want to post my solution in case someone else runs into this issue. I have several email addresses associated with my paypal account and use one particular one all the time so it was the one I used to populate the plugin with. The problem was that this email address may be registered with paypal but it is not the primary email address. Only the email address used as the default email in paypal will allow the transaction to go through.

Hi Mohammed, sorry I missed this reply!
You were right of course. I had the 'Enable validation mode' set to yes in the paypal plugin. Finally fixed all the errors except for error code 10002, but I have just disabled express checkout for now. So the shop is up and running. Thanks for all of your responses and trying to get this sorted for me, it's much appreciated.
Nadja