I have been taught that the uncertainty in the measurement of a metre ruler is +-1 mm. However , I was also taught that the uncertainty is half of the smallest division in the measuring instrument. So, it should be that the uncertainty of the measurement of a metre ruler is +-0.5 mm( its smallest division is 1 mm) ?
There is no "one size fits all" answer to your question.
First - the size of the smallest division on a meter ruler need not be one mm. I have a ruler that only goes down to half cm divisions, and I have one that gives half mm divisions.
Second - a ruler may not be accurate to the nearest division. Wooden rulers in particular will grow and shrink with humidity, they can become bent, and they may have been poorly constructed to begin with. Metal rulers tend to be better in this regard.
Third - your ability to align the ruler with the thing you are measuring. Parallax error can come into play (more so for thicker rulers), as well as a "zero" error: does the end of the ruler really correspond to zero? Is the end fully straight, or worn? Is the ruler accurately aligned with the direction of the thing you are measuring?
Example of two rulers that don't agree on "zero" (by about 1.2 mm) - note also the effect of parallax, where the line of 1" aligns exactly, but the 0.5" and 1.5" lines seem to be shifted; this is due to the relatively close distance of the camera to the ruler, and the magnifying effect this has on the metal ruler relative to the wooden one behind it:
All these factors matter in determining the error of your measurement. But if you are just interested in quoting the number you read off your ruler (assuming it is marked in mm) and you thought the nearest value was 345 mm, then you would have to ask yourself - could it have been 346? If your measurement was "almost half way" between two values the answer is clearly "yes", and you can see you would be wrong to say +- 0.5 mm; this is why one would commonly quote the error due to the device as one unit of the least significant measurement. But note that other factors may contribute additional error.