The reason that your value is negative is that, it depends on which number you put first, when subtracting the accepted value from the experimental actual value, you have to put a | | (absolute sign) around the subtracted values. (that means that the value is accepted as being possitive,even if the value comes out negative because the actual experimental value was smaller than the accepted value.
so it comes out as..
| 2.564- 11.35|/11.35 x 100 = 77.41% error
It can't be negative.
Some profs like to use the absolute value so they always subtract the larger number minus the smaller number (so as to get a positive value) and divide by the accepted value and that times 100. The same rational applies to using the | | to obtain the absolute value as Christina has answered. But to answer your question, yes, there can be a negative value. And some profs like to use the negative value so they ALWAYS subtract it this way.
[(exp value - accepted value)/accepted value]*100 which ALWAYS gives a negative value if the experimental value is lower than the accepted value and ALWAYS gives a positive value if the experimental value is higher than the accepted value. The rationale of those profs is that the sign (negative or positive) tells them immediately if the result was too high or too low. I never worried about that piece of information because if it was in error more than about 0.2% to 0.4%, that was too much for me and I started over. I hope I didn't provide far more information than you wanted.