I don't know what you are doing but maybe
wilcox.test(FvFm0H$D0 ,FvFm0H$D1, paired = TRUE)
but then not sure what add_significance() relates to or is trying to accomplish
I don't know what you are doing but maybe
wilcox.test(FvFm0H$D0 ,FvFm0H$D1, paired = TRUE)
but then not sure what add_significance() relates to or is trying to accomplish