If you are receiving the error message "An FBA offer already exists", this error is caused by Amazon's July 2020 update to prevent ghost listings which happens when inventory is attempted to push to an FBA listing, which only Amazon has control over.

This means that you have an FBA Listing in Amazon but in Extensiv Order Manager, the Listing SKU is set to push inventory.

Proper Amazon Listing Set-Up

You will need to ensure that in Amazon, if your listing is set to FBA, then in Order Manager your Listing SKU is set to Push Inventory = NO.

You should also check that in Amazon, if your listing is set to FBM, then in Order Manager your Listing SKU is set to Push Inventory = YES.