When updating I got the following message:

```
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used.
GPG error: http://dl.google.com/linux/chrome/deb stable Release:
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 6494C6D6997C215E
W: Failed to fetch http://dl.google.com/linux/chrome/deb/dists/stable/Release.gpg
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 6494C6D6997C215E
W: Some index files failed to download. They have been ignored, or old ones used instead.
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Calculating upgrade... Done

```

I'm on Ubuntu 16.04 and currently using Chrome 60.0.3112.90. Any suggestions? I'm also trying to install libimobiledevice but apparently the system is not reading the repository where I just added the PPA as I can't install it. Thanks in advance for any help.