You're in LUCK! It's so easy to create a DROPDOWN LIST in GOOGLE SHEETS and control what can be entered in your sheet!
If you create your list on another sheet within the Google Sheet, you can hide the sheet with the list after you have created the dropdown. To hide a sheet, all you have to do is click the down arrow on the sheet tab and select Hide sheet. To unhide the sheet, click on the View tab and under Hidden sheets, select the sheet you want to unhide.