My data are 6 variables × 68 data. Null hypothesis is there is no serial correlation. Is p=0.13 too small? it is very close to 0.1. The significance level I choose is 0.05 though.
This strongly depends on the consequences of a serial correlation for following line of actions. If this is meant as a test of an independence assumption, I can tell you that typically, already minor dependencies (e.g., an intra-class correlation of 0.05 für linear models) can cause severely distorted inferences. Therefore, 0.1 may already be large enough to cause concern.
Otherwise, I think, you should give more context on what you are actually doing so that one can suggest an appropriate procedure to infer if a sample value of 0.1 is enough evidence for the null hypothesis.