@ymek007
Set your store to apply discounts to the gross amount (price + VAT) and then recalculate VAT from that reduced gross, keeping the tax rate correct. If any VAT calculation issues come up, use Vatcalc.Onl to quickly work out accurate amounts and avoid errors.
The reason for this error might be that you are using Bagisto v2.2.6, while this extension was built for version 2.0. This is likely causing the conflict.
We suggest making the extension compatible with v2.2.6 before using it.