I am using the "org.Ss.eg.db" package to input gene names to my list of ENSSSCG....

sigs <- readxl::read_excel("C:\R\C_vs_F_genes_genelist_Galaxy.xlsx")

sigs.df <- as.data.frame(sigs)

sigs.df

sigs.df <- mapIds(org.Ss.eg.db, keys = rownames(sigs.df), keytype = "ENTREZID", column = "GENENAME")

A message returned

Error in .testForValidKeys(x, keys, keytype, fks) : None of the keys entered are valid keys for 'ENTREZID'. Please use the keys method to see a listing of valid arguments.

Please, help me to solve the problem.

The error message is meant to be self-explanatory. At the risk of being redundant, it says that none of the rownames of your data.frame are NCBI Gene IDs. Without knowing what those rownames are, it's impossible for anybody to help you.