Developer tab is missing in your Excel ?

Default Tabs : Home, Insert, Page Layout, Formulas, Data, Review, & View.

To bring the developer tab in ribbon

Click File > Options

File > Option

Once you click the options, you will get Excel Options window in that select Customize Ribbon.

Customize Ribbon

Right side of the window you can see Customize the Ribbon

If you notice below the Developer tab is unchecked

Developer Tab

Tick the  Developer check box and click OK.

See Also
Add custom tab to excel ribbon