Viewshed Analysis Failing

[GW201]

Hello everyone, 

I'm wondering if someone can help me find the issue as to why I can't preform viewshed analysis in Arc. 

I'm using version 3.1 and the ready to use viewshed analysis tool. 

I get the following message (& lack of error message) when I attempt to use the tool. 

Does anyone have any ideas as to what the issue is?  Any help is greatly appreciated 🙂 

Thank you,

George