I am trying to create a dataframe with all my background genes in order to correctly perform GO pathway analysis. To do this I've used a following code:

all_genes <- bitr(rownames(merge_table), fromType = "SYMBOL", 
                  toType = "ENTREZID", OrgDb = org.Hs.eg.db)
background_genes <- all_genes$ENTREZID

I have obtained a warning message:

In bitr(rownames(merge_table), fromType = "SYMBOL", toType = "ENTREZID",  :
  0.21% of input gene IDs are fail to map...

Is this warning message problematic or can I run my code despite it? What are the possible ways of fixing this issue? I have looked and I do not have any N/A in my code.