The package InstallAssistant.pkg fails to meet Gatekeeper policy.

The Big Sur installer was (is?) a mess for some people & systems. It took me two days to get Big Sur installed to a MBPro 2016 laptop after fighting with cryptic error messages. For anyone having issues running the Big Sur installer from within macOS you should try creating a macOS Big Sur USB installer which should bypass those Gatekeeper error messages:

https://support.apple.com/en-us/HT201372

Or you can try booting into Internet Recovery Mode using Command + Option + R to access the online Big Sur installer:

https://support.apple.com/kb/HT204904

I highly recommend using the bootable macOS USB installer method as it eliminates (or at least minimizes) networking issues.

I also highly recommend you have a good backup before attempting to upgrade to Big Sur in case something goes wrong during the upgrade process or in case you decide you have apps or hardware that is not compatible or you just don't like Big Sur.

Edit: Also make sure to download the most recent version of the Big Sur installer since maybe the latest 11.1 version is better. I used the v11.0.1 installer when I had the issues. Check your Applications folder for the "Install macOS Big Sur" installer and delete it and empty the Trash so you can download the latest version of the installer.

You have to download installassistant.pkg, http://swcdn.apple.com/content/downloads/00/55/001-86606-A_9SF1TL01U7/5duug9lar1gypwunjfl96dza0upa854qgg/InstallAssistant.pkg to your download folder (or wherever you like), then right click it (if you don't have right click enabled on your mouse, hold control key and click the installassistant.pkg icon) this will pop-up a menu (contextual menu) then, in that menu, select "open" (normally the first item in that menu)

If the InstallAssistant.pkg comes from apple, it should open without problem. If not, then you've downloaded an unofficial installer.

with Apple supports help...I shut down my computer and upon turning it back on held the Option-Command-R buttons till it booted to the spinning globe. At the Utilities window selected reinstall Mac OS. Once completed and system restarts. I was able to install Big Sur successfully.

Make sure you back up your system before...just in case.

I had an issue at first when trying to download the installer via chrome, the downloaded package came out as empty, only when I downloaded it with safari the installer worked fine.

Also as previous comments indicate, a corrupted installer could be due unstable internet connection.

Right-Click on the InstallAssistant.pkg, a menu will pop-up, there, select open

Solution to InstallAssistant.pkg Nightmare

After spending hours trying to download Big Sur at least five times while consulting with different techs at Apple Support for hours, my issue was forwarded to a senior technician. We tried a few approaches, which weren't effective until she suspected that my internet speed wasn't adequate and was probably the reason for the InstallAssistant.pkg error notifications and extended installation times. She directed me to turn off my modem or router then turn it on again (20 seconds or so). The reboot apparently increased internet speed and Big Sur was then installed and successfully downloaded!

Afters hours on the phone with Apple Support my tech suspected it was my internet speed not the package. I unplugged my modem and then plugged it in again to reboot which apparently improves the speed. This worked after six attempts to install the upgrade.

Received message "The package InstallAssistant.pkg fails to meet Gatekeeper policy." when trying to install macOs Big Sur. How can I disable the gatekeeper?

When and Where to do this CONTROL-open WHAT?

Can't install macOS Big Sur An error occurred during install...

The package InstallAssistant.pkg fails to meet Gatekeeper policy.

to solve this issue, all you have to do is to be sure that you have a high internet connection whe you’re between 8go and 12go of downloading.

This has happed to me as well four times. Does anyone have a solution? Changed my settings in Security so it isn't so restrictive with regard to apps. Have spent hours talking to techs at Apple Support. Have no idea what installassistant.pkg is or where it is on my computer. Did not understand holding the control key in the "contextual" menu. What does that mean? Very frustrating.

what does that mean

what is the contextual menu ?

is the menu that pop-ups when you right-click something on Mac or almost any other OS