which method is more serious sampling error or nonsampling error and why ?

In stats, a sampling error is simply one that comes from looking at a sample of the population in question and not the entire population. That is where the name comes from. But there are other kinds of stats errors.

In contrast, non sampling error refers to ANY other kind of error that does NOT come from looking at the sample instead of the population. One example you may want to know about of a non sampling error is a systematic error.

OR

Sampling Error: There may be inaccuracy in the information collected during the sample survey, this inaccuracy may be termed as Sampling error.

Sampling error = Frame error + Chance error + Response error.