When you try to enter or change data in a worksheet (worksheet: The primary document that you use in Excel to store and work with data. Also called a spreadsheet. A worksheet consists of cells that are organized into columns and rows; a worksheet is always stored in a workbook.) cell, you see a data validation error alert. This alert signifies that the owner of the workbook applied data validation (data validation: An Excel feature that you can use to define restrictions on what data can or should be entered in a cell, and to display messages that prompt users for correct entries and notify users about incorrect entries.) to the cell to prevent users from entering invalid data, and implemented the error alert to let you know that the data you entered is invalid.
You can enter only valid data in cells that have data validation applied. If you are not clear about the validity of the data that you can enter, you should contact the owner of the workbook.
If you inherited the workbook, you can modify or remove the data validation unless the worksheet is protected with a password that you do not know. If possible, you can contact the previous owner to help you unprotect the worksheet. You can also copy the data to another worksheet, and then remove the data validation. For more information about how to change or remove data validation, see Change data validation settings or Remove data validation.