How to Create a Drop-Down List in Excel

You can help people work more efficiently in worksheets by using drop-down lists in cells. Drop-downs allow people to pick an item from a list that you create.

1. In a new worksheet, type the entries you want to appear in your drop-down list. Ideally, you’ll have your list items in an Excel table. If you don’t, then you can quickly convert your list to a table by selecting any cell in the range, and pressing Ctrl+T.Notes: 
   • Why should you put your data in a table? When your data is in a table, then as you add or remove items from the list, any drop-downs you based on that table will automatically update. You don’t need to do anything else.
   • Now is a good time to Sort data in a range or table in your drop-down list.
2. Select the cell in the worksheet where you want the drop-down list.
3. Go to the Data tab on the Ribbon, then Data Validation.Note: If you can’t click Data Validation, the worksheet might be protected or shared. Unlock specific areas of a protected workbook or stop sharing the worksheet, and then try step 3 again.
4. On the Settings tab, in the Allow box, click List.
5. Click in the Source box, then select your list range. We put ours on a sheet called Cities, in range A2:A9. Note that we left out the header row, because we don’t want that to be a selection option:
6. If it’s OK for people to leave the cell empty, check the Ignore blank box.
7. Check the In-cell dropdown box.
8. Click the Input Message tab.
   • If you want a message to pop up when the cell is clicked, check the Show input message when cell is selected box, and type a title and message in the boxes (up to 225 characters). If you don’t want a message to show up, clear the check box.
9. Click the Error Alert tab.
   • If you want a message to pop up when someone enters something that’s not in your list, check the Show error alert after invalid data is entered box, pick an option from the Style box, and type a title and message. If you don’t want a message to show up, clear the check box.
10. Not sure which option to pick in the Style box?
    • To show a message that doesn’t stop people from entering data that isn’t in the drop-down list, click Information or Warning. Information will show a message with this icon  and Warning will show a message with this icon  .
    • To stop people from entering data that isn’t in the drop-down list, click Stop.Note: If you don’t add a title or text, the title defaults to “Microsoft Excel” and the message to: “The value you entered is not valid. A user has restricted values that can be entered into this cell.”

Ref: https://support.microsoft.com/en-us/office/create-a-drop-down-list-7693307a-59ef-400a-b769-c5402dce407b