The solution I used:
cd /var/lib/dpkg/info/
Remove all insync files:
rm insync.*
Ensure you have the correct GPG key from Insync, as apt-key has been discontinued in Debian Trixie. The Insync documentation is currently incorrect. Please run the following command:
sudo gpg --no-default-keyring --keyring /usr/share/keyrings/insync.gpg --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys ACCAF35C
Run the command apt update and check if the error has disappeared. If necessary, reinstall Insync.