PC-21s with boards 71-003901-02 REV P and newer should use JU4 instead of JU3 for their jumper setting.
The current revision of the PC21 User Guide shows a jumper on Pin JU3. This is correct for older versions of the PC21 boards. The new boards have larger RAM and they require the jumper on JU4 instead. The easiest way to determine if the board requires JU3 or JU4 is to look at the chip located in socket U13. If the socket U13 is full, it has the larger RAM and the jumper should be on JU4. If the socket U13 has empty pins, the board has the smaller RAM and the jumper should be on JU3. If the jumper is set wrong, the RED led will remain lit, the board will not work and PC21term will not start.