imovie no longer works with Big Sur and istore will not allow me to update on MBP.

How can I update iMovie on my new Mac Book Pro (less than a year old). After installing Big Sur, it says that I need to update iMovie, however at the apple istore it says that It is not an app purchased with my Apple ID. How can I update it as the software was already installed when the computer was purchased? Can I uninstall and install from scratch with my Apple ID?