Inherent error in mass and volume?
Is the percent error formula you are looking for something like the formula this website service offers? Your lab instructor should have given you the formula, perhaps you missed that. Do you have a reliable lab partner you can ask? The error in mass would probably be something to do with how far each measurement deviates from the average.

If for example I used a 250 ml Erlenmeyer flask which had marks at only 25ml increments. I would then say that I could make a 'guess' as to what the volume is at if I were trying to measure something less than 25ml or between two marks. But the error for that instrument would then be somewhere around +/- 12.5ml error. My general-chem teacher was fine with that, maybe you should ask yours?

I was told that the convention is to expect at least 2% error in the last significant digit if not explicitly defined. When taking a measurement with a device like a graduated cylinder, you always measure down to the most precise "known" value, and then the last significant digit is your own judgement.