chrome://extensions/ . Check that Developer mode is toggled ON in the top right. If not, toggle it ON.If you donāt see this above option, then open chrome://flags in another page.
Search for "m1" there and enable all unexpire flags. (Change from Default to Enabled)

Relaunch chrome to apply these changes.
Then go to chrome://flags again and search for manifest. The Allow Legacy extension manifest versions option will be available now.
How to Install the Beechine Extension (1).mp4