Sampling tools create points on the network in a variety of ways; more advice on the different techniques that RivEX uses to sample a river network is found here.

All the sampling tools require your network to have had a workspace created, topology extracted and for some of the tools, specific attribution added to the network. Which attribution is required is documented on the tool's help page but tools will halt if the network is missing the required attribute and warn you via the tool tip.

If you have followed the sequence as outlined in Preparing a river network for processing in RivEX then you will be able to sample the river network.