Follow the below steps:
1. Login to Magento admin dashboard.
2. In the Menu on the left, select "System".
3. Click on "Integration".
4. Click on "Add new Integration".
5. Click on "API" and select "Resource access" as "All" from the dropdown.
6. Click on Integration info and Enter Name and Password (admin password).
7. On the right, select "Save & Activate" from the saving button dropdown.
8. Click on "Allow".
9. You will get four values in the last page, copy the "Access token"(Not "Access token secret") value and paste in the input box in Magento Store connection in BigAtom.