No variables available in the Add variable dialog in HMI Editor

Terminal template: most probably there is no device assigned or program mapped at the terminal channel. Add the device to terminal channel by right-click on the channel and select Add device. In the terminal selection dialog, select the device to be added, and click OK.

PLC template: reassign the template by right-click on the template in your HMI project, then select Reassign HMI template and select the PLC.