This error occurs, when the Picklist Value is Not Active. In this case "Webhook API" picklist value was not Active, which throwed an error. Initial actions to solve this is first to check if we have a particular License Options, follow the below steps:

1. Obtaining a License Key                

You can see the pricing for Batch Entry at: https://www.payments2us.com/about/pricing/

To order a license key, please contact support @ payments2us.com

After your order has been processed, you will receive an email with  the license key details.  You need to update your Salesforce instance to  have this key entered against all merchant facilities              

2. Entering your License key and Expiry Date correctly      

In your instance of Salesforce.  Click on the Merchant Facilities Tab.Then Select "All Merchant Facilities" list view filter. For all Merchant Facilities, you will need to press the Edit button,  then scroll down to

the Additional License Options area and update the  License Options, License Expiry Date and License Key as provided in the email notification sent to you.

Even after the above steps,if the "Error" still occurs, then there are chances of Picklists provided not being Active,  In order to make it "Active", please follow the below steps:

Step 1: Click on the Setup Cog button.

Step 2: Click Object Manager and Search for "Merchant facility"

Step 3: Look for "License Options" in fields and relationships.

Step 4: Scroll down to Picklist section and Activate the Deactivated ones, which you want to use as a Picklist Value. In the below image, Webhook API and BPAY CRN are deactivated picklist.