- Make sure logo is of a time image and not say a PDF
- Salesforce changed the way files are loaded up under notes and attachments. Your instance of Salesforce may have had the newer files and attachments type option enabled.
You will need to revert back to the older way (you turn the newer on on after uploading the logo image).
Navigate to setup (top right). Then do a quick find/search for "Salesforce Files" - select the "General Settings" sub menu option.Click edit and UNTICK/deselect "Files uploaded to the Attachments related list on records are uploaded as Salesforce Files, not as attachments" and "save"
Now follow the procedure again - How to put your company logo on the Receipt Template