How are you publishing your web tool ? Are you referencing registered data or copying all data? I encountered a similar error when trying to publish a web tool. The main cause for the error in my case was that the result layer which was created when running the tool in ArcGIS Pro was not accessible anymore (e.g. deleted the result in ArcGIS Pro after I ran the tool). Maybe check where your result layer is saved or create a simple model with an input parameter, the geoprocessing tool and an output parameter.
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