I was trying to test a web service ... facing error while selecting the service

It could be related, but in general the root cause is:

1. Most likely for the reason Aleksi explain in that link Appian handles part of the definition of the enumreation through the creation of a data type called ACHTypes?key (notice that it has ?key as part of the name so don't confuse it with ACHTypes)

{schemas.datacontract.org/.../Ryder.Enterprise.DataTransferObjects}ACHTypes

2. This data type is NOT visible in the Data Management Tab, therefore you cannot delete it to refresh the web service node, and since deleting all CDTs is a requirement you get the error you're seeing.

3. The way I was able to workaround this is by explicitly deactivating this data type (to mimic the "Delete" action in the Data Management tab) through the API with the TypeService.deactivate method.

It could be related, but in general the root cause is:

1. Most likely for the reason Aleksi explain in that link Appian handles part of the definition of the enumreation through the creation of a data type called ACHTypes?key (notice that it has ?key as part of the name so don't confuse it with ACHTypes)

{schemas.datacontract.org/.../Ryder.Enterprise.DataTransferObjects}ACHTypes

2. This data type is NOT visible in the Data Management Tab, therefore you cannot delete it to refresh the web service node, and since deleting all CDTs is a requirement you get the error you're seeing.

3. The way I was able to workaround this is by explicitly deactivating this data type (to mimic the "Delete" action in the Data Management tab) through the API with the TypeService.deactivate method.