Do you mean buying more plots of land? Make sure you start by doing all the essentials:

1. Build a Wood Yard and stock up.
2. Build one of each of the three Tier 1 houses and breed Workers and Level 1 Tenants.
3. Build a Gadget Factory. Depending on your starting location, you may need another plot for this.

Keep in mind that the Council will let you buy another plot if you've used enough (~90%) of your first plot.

The red bar is black marks given to you for failing to resolve complaints lodged by your tenants. If it fills all the way up then you lose your job! Game Over!
You can get white marks by completing some missions to reverse it.

Hello,

Could someone help me please?

I'm struggling to add some of the gadgets to the plots - like gnomes, dog kennels, ponds, sheds

There doesn't seem to be a button to send it with the post man an there isn't any button to call it in from the park lot for example. Keep losing the Park quest because can't install the pond

Any way of fixing it?

Not sure if you're still looking for help but if so, take a look at this screenshot: https://imgur.com/a/5cla5qc

To place gadgets, click on the button circled in yellow, use the buttons circled in orange to select the gadget you want to place and then click on the map where you want to place it.