Hi @chuck.jowett ,
The Error states the dt_FullName is not declared, either you have declared it within a Different Scope and it is not accessible else you are using the Wrong variable as a Different variable is shown in the First Screenshot.
Also, To Add rows to Datatable (If Datatable rows are Empty), you would need to use Add Data Row Activity.
Let us know what if you were able to perform your task using the information provided.