How Do I Fertilize Garden?

If you are using the fertiliser that came with Cottage Living, it only works on oversized crops. Perhaps you may be trying to use it with other plants instead? 

hermettte  In addition to what rosemow said, the only fertilizer in the game that you can fertilize garden plants with is from the Eco Lifestyle bug boxes where you have beetles (2 types), crickets and grubs.  You have to nurture them to get the fertilizer. Otherwise, you can fertilize your garden plants with fish and pretty much every other harvestable plant produce, including flowers.  Some of the things you can use work better than others though.  There are online guides that you can find by doing a search with your internet browser that cover that.

Hope this helps.

Thank you. Yes, it does help. I don't recall seeing the Eco Lifestyle bug boxes. I've only begun playing again after years of taking a break. Now, everything is so different and because of changing computers several times, I needed to buy and download all over again. Perhaps they are in an upgrade I haven't downloaded yet. Thanks again. I wasn't aware of the flowers and such. She has containers saying various fertilizers in her storage, so thought I was missing something as to why they didn't pop up in menu. 😉

Yes, I'm in the Cottage Living house. You bring up another question, sorry but, I see the oversized crop seeds but am lost as to how or what to plant them on. Seed boxes don't work, nor so the round ones. Sorry, I've been away from Sims for a few years, and everything has change so much. It's exciting, yet overwhelming. Thank you for your help.

This is a link to a guide on growing over sized crops.  They need to be planted in the gardening plots that come with Cottage Living (which are found in buy mode). It is great that you are back simming again after a break. Happy simming! 
https://simscommunity.info/2021/07/26/the-sims-4-cottage-living-oversized-crops-guide/

hermettte  You have to have the Eco Lifestyle expansion pack to have the bug boxes.  You can find them in the build/buy catalog.  

And you shouldn't have to buy any pack you owned when you stopped buying them again as long as you haven't changed platforms.  Those are all tied to your EA Account and once you get the EA App up and running and log in with the same username and password you used before, you can get the base game and all the packs you own downloaded all in one go.  How long the install takes can vary depending on your internet download speed.  The only thing you'll need to buy are the packs you don't already own.  That said, if you switch platforms, say from PC or Mac to a console, you will have to buy everything new or vice versa.  You won't have to buy the base game new though as that is now free to everyone on all platforms.   

Hope this helps.

Thank you again for your excellent advice. Much appreciated.