This usually means one of two things:

• Opening the file directly (file:// URL) — browsers block local file loading. You must serve the folder via a web server, or use the Python trick below.
• File missing — confirm CT26_Submissions.xlsx is inside the assets/ folder.

Quick local test: open a terminal in the ct26 folder and run:
python3 -m http.server 8000
then visit http://localhost:8000