The version of the server that will host the web tool you are publishing does not have this geoprocessing tool.
If the publishing application (ArcGIS Pro) is a newer version than the server software (ArcGIS Enterprise), and the tool is new to the version of ArcGIS Pro, it will not exist on the server.
Solution
Update ArcGIS Server to a matching version of ArcGIS Pro or later.
If upgrading ArcGIS Server is not possible, you will need to remove the tool from your workflow and find alternative tools to achieve your desired workflow.