Eventually after some time, according to the expiry duration you selected when generating client ID and secrets, LHDN will send you this email :
This is a normal part of the e-invoicing process, not to worry.
To explain,
When the client ID and secret was generated, a selection was made for an expiry date for the Secrets:

This is a security precaution set up by LHDN to ensure constant activity and conscious renewal of ERP systems with LHDN.
What this means is that either before or when the Client Secret are going to expire, as per the instructions by LHDN, you will need to : Login to your LHDN MyTax portal,
Click "regenerate secrets"
Note: you will ahve a new option to select a new secret expiration date,
you may change this to whatever interval available you desire.
Once you generate these new secrets, IMMEDIATELY replace the secret in Zoho Books Plugin Config,
this is because your old secrets will not be valid anymore upon secret regeneration.
With this you will have new client Secrets to use with Zoho until the next expiration date.
You will need to replace the old client secret in the plugin config page as soon as these new secrets are generated as the old ones will no longer be operational.
Once you update the Client secrets to the new one in plugin config page, you may save and proceed with e-invoicing as normal.
Please do not hesitate to contact us if you have any questions or require any assistance.