Pay CGT Online - File Upload Error

I am trying to pay my CGT on a shares sale in the 2023/24 Tax Year. I am required to give evidence of my calculations and, as instructed, have created a simple Excel spreadsheet. I've have tried uploading this file a handful of times - both from my iPad and a laptop - but I keep getting a "File Upload" error. No explanation of what has gone wrong or what I need to do to fix it.

Hi,
You will need to save the file in PDF format.  
You can do this in Excel by clicking 'File' and selecting 'Export'.  
You should now be able to attach your evidence.
Thank you.

I had worked out the the upload did not like Excel spreadsheets. Created a PDF file and successfully loaded this file. Problem solved.