This error means that the formula in the cell doesn't match the pattern of the formulas nearby.
Troubleshoot why it's inconsistent

Select Formulas > Show Formulas.

Doing that lets you see the formulas in all cells, instead of the calculated results.

Compare the inconsistent formula with ones before and after it, and correct any unintentional inconsistencies.

When you're done, select Formulas > Show Formulas to show the calculated results of all cells.

If that doesn't work, select a cell nearby that doesn't have the problem.

Select Formulas > Trace Precedents.

Select the cell that does have the problem.

Select Formulas > Trace Precedents.

Compare the blue arrows or ranges in blue, and then correct any problems with the inconsistent formula.

Select Formulas > Remove Arrows.
More solutions
Select the cell with the inconsistent formula, and then hold down the SHIFT key while pressing one of your arrow keys. This selects the inconsistent cell along with others. Then do one of the following:

If you selected cells below, press CTRL+D to fill the formula down.

If you selected cells above, select Home > Fill > Up to fill the formula up.

If you selected cells to the right, press CTRL+R to fill the formula to the right.

If you selected cells to the left, select Home > Fill > Left to fill the formula to the left.
If you have more cells that need the formula, repeat the above process going in the other direction.

Select the cell with the problem.

Select this button: and then select either Copy Formula from Above or Copy Formula from Left.

If that doesn't work and you need the formula from the cell below, select Home > Fill > Up.

If you need the formula from the cell to the right, select Home > Fill > Left.
If the formula is not an error, you can choose to ignore it:

Select Formulas > Error Checking

Select Ignore Error.

Select OK or Next to go to the next error.
Note: If don't want Excel to check for inconsistent formulas like this, close the Error Checking dialog box. Then go to File > Options > Formulas. At the bottom, uncheck Formulas inconsistent with other formulas in the region.
If you're on a Mac, select Excel > Preferences > Error checking, and then uncheck Formulas that don't match nearby formulas.
An error indicator appears when the formula does not match the pattern of other formulas near it. This does not always mean that the formula is wrong. If the formula is wrong, making the cell reference consistent often solves the problem.
For example, to multiply column A by column B, the formulas are A1*B1, A2*B2, A3*B3, and so on. If the next formula after A3*B3 is A4*B2, Excel identifies it as an inconsistent formula, because to continue the pattern, the formula should be A4*B4.

Select the cell that contains the error indicator and look at the Formula Bar to verify that cell references are correct.

Select the arrow next to the button that appears.
The shortcut menu shows the options available to resolve this warning.

Do any of the following:
Select 
To 

Copy Formula from Above 
Make the formula consistent by copying the pattern from the cell above. In our example, the formula becomes A4*B4 to match the pattern A3*B3 in the cell above. 
Ignore Error 
Remove the error indicator, for example, if the inconsistency in the formula is intentional or otherwise acceptable. 
Edit in Formula Bar 
Review the formula syntax and verify that cell references are what you intended. 
Error Checking Options 
Select the types of errors that you want Excel to flag. For example, if you don't want to see error indicators for inconsistent formulas, clear the Flag formulas that are inconsistent with formulas in adjoining cells check box. 
Tips:

To ignore error indicators for multiple cells at a time, select the range that contains the errors that you want to ignore. Next, select the arrow next to the button , and on the shortcut menu, select Ignore Error.

To ignore error indicators for a whole sheet, first select a cell that has an error indicator. Then press + A to select the sheet. Next, select the arrow next to the button , and on the shortcut menu, select Ignore Error.
More resources
You can always ask an expert in the Excel Tech Community or get support in Communities.