First, NEVER Uninstall an IOS BELOW 200! Never. The reason you get the error... you already uninstalled IOS15 so it wont uninstall it again.
For your second question regarding loading backups. Read the Intro Post I gave you. It has the links and Info you need.
Post a syscheck and we will go from there...(Regarding the IOS Issue)
How to Run a Sys-Check -- Simple Instructions