How to fix "identity of developer cannot be confirmed for libreoffice"

downloaded liver office but it will not open

What is your OS (including version)? If you downloaded your installer from the official site (www.libreoffice.org) there should be an issue with your security setting on system level.

Sign up for a user account! https://ask.libreoffice.org/en/account/signin

OP clearly has an account as they have successfully posted here.