Right-click the Modbus Poll shortcut → → Compatibility → check "Run this program as an administrator" → Apply. If the error disappears, you need to permanently elevate privileges or add your user to the "Dialup" group (old school).
Disconnect your device and short the pins on your converter (pins 2 & 3 for DB9; A to B for RS485 with a resistor). Open Modbus Poll → Connect → select your COM port → baud rate 9600. Send any query. If you receive a framing error or no echo, the port opened successfully. If you still get "Port not available," the OS is the problem. modbus poll port not available
For Modbus/TCP connections: A security suite may block the application from binding to a TCP port, especially if it’s a privileged port (below 1024). Right-click the Modbus Poll shortcut → → Compatibility