If I have a rogue, can I use expertise on Dex saving throws?
Can I check off saving throw for expertise instead of a skill? Both are in the same area of my character sheet.
I can honestly understand your confusion; I've been there before.
Generally, in 5e, you will have three basic types of rolls that the player will be asked to make: the attack roll, the ability check, and the saving throw.
Ability checks and saving throws both use your ability scores (and they do appear in the same area on your character sheet), but they are very different things mechanically.
Choose two of your skill proficiencies.
Since it specifically says skill proficiencies that means that saving throws cannot be applied instead (because they are different).
Reading the entry for each in the Player's Handbook (Chapter 7) should help clear up the exact differences between the two. You can access the relevant sections online even: Ability Checks (Skills) and Saving Throws.
Note: You can also choose to apply expertise to a tool proficiency that you have, but that isn't strictly relevant to what you asked.