Trying to install OS MONTEREY but getting "null"

Has the user attempted to download Monterey more than once ?

If so, check in Applications folder for " Install Monterey " and drag and drop to Trash and Empty Trash.

Try again to download Monterey only this time restart in Safe Mode and attempt the download via the Apple Apps Store.

See the following for null error when updating:

https://developer.apple.com/forums/thread/679068

checking "(null)"... and stuck - Apple Community

https://www.reddit.com/r/MacOS/comments/pkzrq2/update_error_checking_null_i_cant_get_my_macbook/