Select the “Review” Ribbon Form the Top of Your Excel Sheet
Select the “Check Accessibility” Button
On the Right Side the Results will Come in
When You Select Any of the Results It Will Bring You to The Issue
The green box covering the selected issue with the chest around the merged cells would show that this is what the accessibility checker is referring to as an issue within the sheet.
The issue here is that there are merged cells which makes it harder for a screen reader to read this would go against the best practices for making an excel sheet. You can find more information about this with the following article Best Practices to Allow Excel Sheets to Be Most Accessible
Some of the issues might include some of the formatting that is included in the How to Add Alternative Text to Excel (Desktop Mac) or missing alternative text which can be found How to Add Alternative Text to Excel (Desktop Mac).