There are two possible situations where you might see this message:
You see a green triangle in the upper-left corner of a cell that contains a formula.
You are evaluating errors in the Error Checking box.
By default, all cells are locked to protect them from accidental or unauthorized changes. In this case, the cell containing the formula is not locked for protection.
Lock the cell by doing one of the following:
Click the Error Checking button next to the cell, and then click Lock Cell.
In the Error Checking box, click Lock Cell.
Protecting cells that contain formulas prevents them from being changed, and can help avoid future errors. However, locking the cell is a first step, and you must perform additional operations to protect the workbook, such as setting a password.
For more information about how to protect a workbook, see Protect a worksheet with or without a password.
Note: You can leave the cell unlocked if you prefer to leave it unprotected. However, the message will continue to appear unless you disable this error checking rule.
For more information about how to control error checking, see Detect errors in formulas in Excel 2016.