You have not provided any indication of the actual temperature, so we cannot compute the error in a reading of 23°.
The error is computed from
.. ((measured value)/(actual value) -1) * 100%
.. (23/(actual value) -1) * 100%
If the actual value is 25°, for example, then the error is
.. (23/25 -1) * 100% = -8% . . . . . . . the reading of 23° is 8% lower than the actual value