@mac01333 Are you using the technical preview (formerly the beta version) of Origin? Something weird may be going on with that. To opt out, hover over your username, select Application Settings, and under Application (the first header), scroll down to Client Update, and disable the option to participate in technical previews. You'll need to restart Origin for the setting to take effect.
If this isn't the issue, or you want to keep using the preview, try buying the pack directly from the Origin app rather than through the in-game interface. You can click on the Sims 4 icon in your Origin library, click More > Expansion Packs, scroll down to where the pack you want is listed, and click Buy Now. You can also start the Seasons download the same way; the orange button below the pack icon should say Download if you own it but don't have it installed.
By the way, the lock icon next to the settings is normal if you're in-game. You can quit Sims 4, maybe quit Origin and relaunch too, disable and reenable Origin in-game, and see whether buying from within the game works. I'd still prefer to go directly through the Origin app though: it's more reliable and less prone to secretly double-charging people.