installer info on recovery server damaged

Look in the Applications folder for Install MacsOS Sierra App, if there delete it & gwt a new copy.

So you are pretty sure the Mac came with Sierra? If it came with anything older, you will need to boot with command-option-shift-R if it is newer than 10.7's 2011 release and then upgrade to Sierra.

If it is older than that you may have to find the original OS installer discs that came with the Mac.

If the Mac only had High Sierra, you won't be able to install Sierra on your Mac.