DO NOT FILE I-90 - YOU ARE NOT ELIGIBLE IF YOUR I-751 IS PENDING!!!
I don't understand why this incorrect advice keeps being given, but in the instructions for the I-90 it says "Conditional permanent residents may not
use this application to replace, for any reason, an existing Permanent Resident Card that is expired or will expire within 90 days.
https://www.uscis.gov/sites/default/files/document/forms/i-90instr.pdf
"Upon receipt of your properly filed Forms I-751 or I-829, U.S. Citizenship and Immigration Services (USCIS) shall extend your conditional permanent resident status automatically, if necessary, until such time as USCIS has adjudicated the petition. The Form I-797 Receipt Notice for such a pending petition will serve as your proof of conditional permanent resident status."
If you need it, you can apply for a stamp at a USCIS location.