The 'Oil pressure low' message presumably relates to the engine oil, not to the Selespeed. When you say that you just topped up the oil, you didn't say whether it was the engine or the Selespeed oil that you topped up. Hopefully if you have topped up the Selespeed, you have used the correct Selespeed oil (not ATF).
I think you have a problem with the two gear position sensors on the Selespeed (though, it could also be mechanical maladies within the gearbox). The gear position sensors are small black plastic items, each with a three-pin plug, bolted to the top and the back of the transmission. If you have a multimeter and know how to use it, you might like to take the sensors off and test them. Something else you can try is to swap the two sensors and see if that makes any difference (see if that allows you to select other gears or otherwise changes the problem).
A couple of weeks ago, I faced a similar problem (not getting gears and skipping gears) with a 147, though oddly enough it worked perfectly once an EOL (End Of Line) calibration was performed. One sensor had an erratic resistance, so I pried open the sensor and cleaned it, and the problem hasn't returned since.
If you do intend to work on Selespeed yourself, have you heard of Alfadiag? AlfaDiag Alfa Romeo Diagnostic Software
Try some 'Selespeed' searches on this forum to find out about it.