Methanol and its density is 791 kg/m3 at 25C. What is the percent error?
Your question as written doesn't make sense. The density of any given liquid is a measured quantity using highly sophisticated equipment with almost no measureable error. The value stated for the density of methanol in a professional source is what is called the "accepted value." If you were to measure the density of methanol in your own laboratory, you could compare your measured value to the accepted value by calculating the percent error. The formula for percent error is:
(measured value - accepted value)/accepted value * 100=percent error
For example, if you used a volumetric cylinder and balance to measure the density of methanol and calculated it to be 793 kg/m^3, the percent error for your value versus the universally accepted value (791) would be:
(793-791)/791 * 100=0.253%
Since you only give one value for density in your question, it is impossible to calculate a percent error.