hi caitlin
to find the master pages and page layouts setting, we need to turn on custom script. please go to sharepoint admin center> settings. turn on custom script as the following shows.

note: it takes about 24 hours for the change to take effect.
if the issue persists, i’d like to collect the following information:
- a screenshot of your permission on the site level.
to check the permission on the site level, please follow the steps below:
click gear in the right upper corner > site settings, in the users and permissions section, click site permissions. in the new page, click
check permission in the ribbon, input the user’s name and click check now.
note: to upload screenshots, please click use rich formatting in this thread and select the insert/edit media icon.
- a screenshot regarding “the only options i have are site columns ad site content types”.
regards.
spike