Let me pick up on where Chris left off. To clarify a few items -
The CreateMethodAccountV3 is utilized by our Partners to create new Method Company Accounts. Based on the previous posts, it does not sound like this is the method you want to be using. Rather, you should be utilizing the MethodAPIInsertV2 to insert a new customer record into your Customer table.
In order to update a Lead to a Customer, you can utilize the MethodAPIUpdateV2 call -
In regards to your error when converting a Lead to a Customer - I have a few questions / troubleshooting tasks
1) Does the error happen for all Leads, or do you only receive this error for a particular lead?
2) I'd like you to regenerate the Customer Table within your Method account. To do so, log into Method and go to the Customize Tab -> Tables screen. Open the Customer Table and click on the 'Finish Editing Fields' button on the bottom right. Then try the API call again to see if things are successful.