You might have encountered this when trying to add the SPF and DKIM records in your domains DNS settings.

You can easily overcome this by checking if there's an existing SPF record in the DNS settings.

To identify a SPF record, check if it contains v=spf1.

If there's an existing record, you can just include "include;sender.zohobooks.com" to it. 

After doing this, login to Zoho Books and validate it.