Some genes that resulted from my differential gene expression analysis don't have corresponding ENTREZ ID and I want to perform KEGG pathway analysis but most tutorials analyze data with no missing ENTREZ ID
Were the annotations to the gene IDs result in hypothetical protein? Did you use KEGG mapper? If not you can find relevant EC numbers or use uniport id retrieval (https://www.uniprot.org/id-mapping). If nothing works, you can search for homologus genes and their IDs try to map your pathway.