Hey everyone, I tried the same thing. Downloaded the new update and tried to install. I had about 4GB of space available after the download. I tried repeatedly to install the same but to no avail. I came upon this discussion and it was helpful. The post by AshishRajpal91 was the one to go with. I deleted a few apps and had about 6GB space and then tried again. I am now posting this reply post installation of ios 15. Have a nice day guys.