Reason for Quantum message : Required "programming" connection mode is not available. Do you wish to use "Monitoring" connection mode instead ? (PLC is already reserved by entity: .)

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The reported error comes when the CPU is already connected with other programming terminal.
Check if the cpu is connected with any other machine with unity pro.

If this is not the case then switch-off the CPU and plug it out from the rack and re-install again and try to connect if it permits as all connections get reset.

Cycle power on the PLC and reconnect.

Something similar may occur if you are using non-original Unity software or if all HF updates are not installed in it.