can i back up my mac pro when i cannot power up mac book also turn off find my device before repair ?

You can turn off "Find My" by using another computer to connect to your iCloud account. We've found it can be very difficult getting a device turned off using the iCloud.com website. Sometimes it works and sometimes it does not. Sometimes it takes multiple attempts to turn off "Find My".

No. The time to back up is before the computer has problems. The purpose of a backup is so that you have a backup before the computer has problems.