For those trying to help,
Please take note of which server board a person is posting in.
For those asking,
In order to claim a plot on Creative, you must be either on the Plotworld (/plotworld) or LargePlots (/largeplots) and need to find an empty plot. You can either walk around til you find one to claim with /plot claim (while standing in it) or you can automatically be assigned to an empty one with /plot auto. Once you've claimed a plot like this, you will be able to build.
Additionally, if you need more space, you can go to a regular world (/world) and start building there. Keep in mind you will need to protect your space yourself on the open world by using a gold shovel in each corner of your build space. The max protection size here is limited, but increases with time.