Set your security token so that eBay identifies your account. Do that by:

1. Go to Settings -> eBay account
2. Make sure you log out of all your eBay accounts opened in the browser
3. Tick the option where it says “I have logged out from all existing eBay accounts…”
4. Click on “Change/Update” button.
5. Sign in to your eBay store account
6. Press “I agree“
7. Your security token has now been set/reset