When I run Openmc v0.13 pincell example, I see this error output, how can I solve this problem?
This means that one of your materials contains O18 but your cross section library does not have data for O18. The most likely scenario is that you are using ENDF/B-VII.1, which lacked cross sections for O18. The easiest fix would be to download and use the ENDF/B-VIII.0 library here. Alternatively, you can make sure that at the time you are generating your materials.xml, you have set the OPENMC_CROSS_SECTIONS environment variable to the library you are using, which will ensure that whenever you call material.add_element, it will only use isotopes that are available in that library.
Full information about cross section configuration can be found here.