Let me get the help you need to ensure you’re able to attach a file to your Bill.
Upon attaching a file, make sure that the file you are trying to attach is one of the supported file types in QuickBooks Online. These are:
PDF
JPEG
PNG
DOC
XLSX
CSV
TIFF
GIF
XML
If the file you are trying to attach is not one of the supported file types, then that is the reason you cannot attach a file. However, if the file is one of the supported file types, have you encountered any error message? If yes, can you please provide more details so I can further assist?