Hi Jessica,
There was a support spam post in this thread, please do not call the number they provided.
I just checked the same Excel version as you mentioned, but the data validation button is listed on my ribbon.
The first thing you need to check is whether you have disable the data validation button in the customize ribbon option. Please go to File > Options > Customize Ribbon, change to all commands, find Data Validation button and add it to the ribbon.
Or, if you haven't made any customization on your ribbon, you could just check the reset button to reset your ribbon to the default settings, see if that helps.
Regards,
Alex Chen