If you buy a new laptop with a preloaded operating system, there is 30 days' trial office 365. If you want to use more than 30 days, you need to buy office. But after removing the office 365 and install new office 2013 or 2016 or 2019, it will still say buy or enter a product key. In this situation what you will do? Today I am going to show you, how to fix this issue.

run regedit
HKEY_LOCAL_MACHIN/SOFTWARE/Microsoft/Office/16.0/Common
Delete OEM file
HKEY_LOCAL_MACHIN/SOFTWARE/WOW6432Node/Microsoft/Office/16.0/Common
Delete OEM file