Are you trying to e-file your 2019 tax return? If you did not file a 2018 tax return at all, then use "zero" for the 2018 AGI. If zero will not let you e-file 2019, you will have to print, sign and mail 2019 instead of e-filing it.
Or are you trying to prepare a 2018 return? "I didn't work" does not necessarily mean you had no income---did you have any other income for 2018? Did you receive retirement income, Social Security, etc.? Please provide some details so we can understand what you are trying to do.