This stuff is really strange at first but it's quite simple. Four steps.
1. Write down the MEASUREMENT.
2. Write down the PRECISION ... how accurate is the measuring device? How many decimal places? To nearest whole number? To nearest 10? Stuff like that.
3. Write down the ABSOLUTE ERROR ... which is ALWAYS half the precision.
4. Work out the PERCENTAGE ERROR ... which is Absolute Error ÷ Measurement x 100
So, for example, if you measure your weight on a set of digital bathroom scales and they give the weight to the nearest kg. Let's say your weight is 77 kg.
1. MEASUREMENT = 77 kg
2. PRECISION = 1 kg
3. ABSOLUTE ERROR = 1/2 x 1 = 0.5 kg
4. PERCENTAGE ERROR = Abs Error ÷ Measurement x 100
= 0.5 ÷ 77 x 100
= 0.649%
I'm not sure what you mean by "maximum error".
In this example, the weight could be 77±0.5 kg. In other words the MINIMUM possible weight is 77 – 0.5 = 76.5 kg and the MAXIMUM possible weight is 77 + 0.5 = 77.5 kg.
What this means that any weight which is ACTUALLY anywhere between 76.5 and 77.5 kg will be rounded off by the bathroom scales as 77 kg.
Maybe this helps.