So in a recovery environment often times the drive letters are changed. It could be that your normal c:/ drive is now d:/
I found the utilman.exe file on the drive where Windows is actually installed — but when you're in the recovery environment since the drive letters are all shuffled. What worked for me was opening Command Prompt from the recovery USB and running this command to find the correct drive:
for %i in (C D E F G H I J K L M N O P Q R S T U V W X Y Z) do @if exist "%i:\Windows\System32\utilman.exe" echo Found on %i:\
Once I saw the correct drive (in my case it ended up being D:), I was able to navigate to D:\Windows\System32
Then you can use the dir command to list the contents of the directory and verify it's actually there. It may be possible you or someone else renamed if previously so listing the directory contents may be helpful
Hope that helps!