There could be a few reasons why QuickBooks is not allowing you to upload the file. Examples: incorrect formatting, file size, limitation of rows, etc. To properly assist you, what kind of information are you trying to import into QuickBooks Online? (customers, bank transactions, chart of accounts, etc) If you're importing bank transactions, there are certain factors to consider.
Make sure the file is on a CSV (comma-separated values) format.
Ensure to save your data as CSV or Comma delimited.
Check the file for any blank areas. If there are some, delete them. Don't want any unnecessary spaces.
Here are two additional links that can provide additional information about importing transactions: