Please provide the minimum amount of information it takes to help you in your initial post upfront such as the OH version you’re running. It is very annoying if helpers have to ask for all the details
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There is no “modbus 1.x” build, there’s only openHAB versions. All “1.x” bindings (those to require a .cfg file) are no longer available in openHAB 3 so you would need to migrate your config to use the new modbus binding that was introduced with openHAB 2.