It's possible that the dataframe is too large to export in-memory, which is one possible cause for not seeing the "Export this dataframe" button.
You can see more information if you open up your console developer tools and then attempt to view your dataframe. You'll see in your browser console some useful information starting with "Checking dataframe exportability". You will likely see some information after this line that tells you more about why the export is not possible. Feel free to attach the output if you need any help interpreting it.