Behavior:
When configuring a Web Service step in a Workflow, the designer Validation displays an error after exiting the step:

Solution:
When this error is prompted, it indicates that one of your assignments in the Web Service step still needs to be completed.
For example, if you are assigning data to a table field in the Data Assign section, it is possible the configuration is not complete:

Even if you have everything selected in the main section, click the Pencil icon to access additional configuration options. Within the Column assignment window, please designate which columns you want to write to:

Once all required assignments are completed, the validation error will be resolved.

KBA applicable for both Cloud and On-premise Organizations
When configuring a Web Service step in a Workflow, the designer Validation displays an error after exiting the step:

Solution:
When this error is prompted, it indicates that one of your assignments in the Web Service step still needs to be completed.
For example, if you are assigning data to a table field in the Data Assign section, it is possible the configuration is not complete:

Even if you have everything selected in the main section, click the Pencil icon to access additional configuration options. Within the Column assignment window, please designate which columns you want to write to:

Once all required assignments are completed, the validation error will be resolved.

KBA applicable for both Cloud and On-premise Organizations