Apple yesterday released the first developer beta of macOS Monterey, there are under the hood changes to this new macOS version that mean Bartender 4 does not work perfectly straight away. I am working on supporting macOS Monterey already, and hope to have a version of Bartender 4 with support for macOS Monterey available as soon as possible. This may take a few weeks or more.
Until then I have released a quick patched version of Bartender 4 with features disabled that are not yet compatible with macOS Monterey, this will allow you to still use the other features of Bartender. You can get the Monterey Developer Beta Patch by checking for updates in Bartender 4 on macOS Monterey, or via direct download here.