You have misunderstood the concept of sampling error. Sampling error is the error that is incurred when the statistical characteristics of a population is estimated from a sample of the population due to the choice of sample.
As a concept this is distinct from the standard error, which you understand correctly.
To demonstrate the distinction a little more clearly, consider a population that contains a single member with some characteristic
C. Now imagine that you wish to measure the average
C within this population. As the population only contains a single member, only one sample is possible, the sample that measures the single existing member. As such there can be no sampling error due to there being no choice in the sample to take.
Despite this it is still be possible to take repeated samples from this population, each of which could have its own unique 'measurement' error. As such these repeated measurements could produce differing estimates of
C and as such have a standard error greater than 0.