USB-to-RS485/RS232 converters (especially Prolific PL2303 and older FTDI chips) have notorious Windows 10/11 driver problems. A generic Microsoft driver may install, but it will report the port as "available" while actually failing to grant proper permissions.
: Missing or outdated drivers for your serial converter can prevent the port from appearing in the software's dropdown menu. 2. Ethernet Communication (Modbus TCP) Issues Modbus Poll User Manual modbus poll port not available