The mandatory fields that need to be filled in the GSTR 1 file for a successful upload include the GSTIN, invoice details, taxable value, tax amount, and HSN/SAC codes. Additionally, consider the following aspects:
1. Ensure that the GSTIN is active for all registered parties.
2. Enter the character limit of HSN/SAC codes based on turnover.
3. Verify that the sale type, such as central or local, is accurately mentioned in the sale voucher.
4. Before generating GSTR-1, thoroughly check possible error reports, including transaction errors, HSN summary errors, and document summary errors. Addressing these issues beforehand will contribute to a smoother and error free filing process.