Android does not allow CSV imports directly anymore, and it mostly supports VCF files. I had the same issue that Android refused to import my CSV even though it seemed to be fine. The main cause of the problem was that Android Contacts requires a file formatted in vCard, not a spreadsheet. The optimal method is to change the CSV file to VCF beforehand. What helped me was the CSV file header cleanup and making sure that the fields matched the standard vCard fields such as FN, TEL, EMAIL, and ORG. After the conversion was completed with the right mapping tool (I used DataVare CSV to vCard Converter for that part), the VCF file was imported in a flash without any error messages. Additionally, make sure that your CSV does not have any empty rows that Android will not be able to process.
Learn more: https://www.datavare.com/blog/export-csv-file-to-vcf-file-format.html