Error apt update on ubuntu help

[Noppliees]

apt update
Get:1 https://termux.net stable InRelease [1720 B]
Err:1 https://termux.net stable InRelease The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 6700B77E6D8D0AE7
Reading package lists... Done
W: GPG error: https://termux.net stable InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 6700B77E6D8D0AE7
E: The repository 'https://termux.net stable InRelease' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details
How to solve this ? I can't update ubuntu.
I use android 11.