I know all the meanings of the word but i don't know how to use the noun in sentence. "You can't do a six years work span in one year" Is this sentence correct? What i mean is that you cannot do six years worth work in one year.
The idiomatic phrase you are looking for is
You can't do a six years worth of work in one year.
using span would not be appropriate in your sentence, if you used "in the span of one year" it would be understandable but redundant, or if you used "six year span of work" you would be referring to the time span of work, not the amount of work.