When using a Web Request action an error may occur when running the workflow.
Some web services or API's do not require a Body in the request as all information is passed in the URL but the Web Request action does need something in the Body even though this section of the action configuration is not starred as mandatory. Adding the content type to the field will resolve the issue even though the content may be ignored by the web service or API.
In general, any error in O365 that has a field name in between square brackets e.g. [Field] means there is an issue with that field. Sometimes the field name is obvious and other times it is not. Read the error to determine the issue. In this example the field is empty "null" but other errors may be in regards to the syntax of the information provided in the field.
Some parameters failed the validation check. - [InputFileContent] : The parameter or the parameter value is null.