Cannot add PTR record to VM instance

Check whether the user which verified the domain ownership and the user who is trying to add the PTR to the instance are the same.

You can check the user verification details at https://www.google.com/webmasters/verification.

Run this command to check the user in google cloud console:

gcloud config get-value account

If both the users are same then try the following steps:

Go to Google Webmaster Central--> Add property-->PTR domain name-->domain verification - "Alternate method"--> domain name provider (then copy the txt record it provides to your domain DNS zone records), once verified then,

Now, In google cloud console-->vm instance--> Edit instance--> nic0 --> public DNS PTR Record --> add your "domain.com" (exactly as shown in the verification result)

Make sure the domain name in Google Cloud "Public DNS PTR Record" should be the same as the host name.