LHS stands for Left Hand Side, and you are getting that error message because as I told you on the previous post you are eliminating the joining key (i.e. Index column) thus the joint can't be executed, If you need to do this steps do it after the joint.
Thank you very much Andres, I solved this problem by renaming the SampleID column by index in the Family database. I have one more question, Andrés, it is possible to extract the file generated from this in Excel format:
Family %>%
left_join(Metadata %>% select(SampleID, Month), by = c("Index" = "SampleID")) %>%
gather-...................................................
Because I don't know how to program the way you do it, using%>% (your scripts are more simplified). And I would like to open the format that was generated and then use my script.
You are right in what you mention but all this has been a challenge for me, that if they ask me for proof of the analysis, I will choose to send them this and the documents in excel as evidence of such analyzes.