You have a few coils such as Coil:Cooling:Water and Coil:Heating:Water somewhere in your model that are likely connected to a PlantLoop yet they aren't connected to any air-side systems such as an AirLoopHVAC (or a ZoneHVACComponent).

Had you shared your model I could have pinpointed them. In the OS App, go to your PlantLoop(s), and look for Coils on the demand side that aren't connected to anything.

The reciprocal is also very likely. If you have dragged an AirTerminalSingleDuctVAVReheat with a HW coil from the library to your AirLoopHVAC demand side, click on the ATU, go to the "Link" icon, and select the PlantLoop to conect it to.