If I have an LLC but do nothing with it and made no income in 2023, what boxes do I check?

If you are the sole owner of the LLC, then you report the income on your personal tax return on a schedule C. If you didn't have any income in the current year you can mark the business as being disposed of provided you don't want to enter anything for it in future years. Otherwise, you can just leave it alone for the current year, provided you don't have any current year automatic expenses such as depreciation. If you have anything like that, you will need to remove the associated asset or make other adjustments to prevent expenses being deducted for a year you were not in business.

You would mark the business as being disposed of by first editing your General Business Information under your Business Profile. 

Then, on the next screen edit the Disposed of Business in Current Year option to read "yes"

If you have a multi-member LLC and file a partnership return, you can file a zero return and/or mark it "final". You are technically required to file a return for any year you did business or had assets.