The SPF and DKIM Signature fields are missing in the email header when an email is sent from one email ID to the same email ID. What is the reason for this?
DKIM headers are usually added by some but not all mail servers for outgoing mail, i.e. mail leaving the domain. SPF headers are added by some but not all mail servers for external mail entering a domain. In your case the mail is only sent from one account to the same account so it probably never passes an outgoing nor an incoming mail server and thus these headers don't get added.
As a general answer:
These ensure the Sender is properly authenticated by the outgoing mail-server.
SPF is currently almost a standard whereas DKIM is yet to become one.
As for your question:
The SPF and DKIM signatures are missing in the email header, when an email is sent from one email ID to the same email ID. What is the reason for this?
There may be multiple reasons, among those are usually Spam, or simply misconfiguration on the server's side.
For example I had no SPF record some weeks ago on my own domain's email, I had to manually configure the DNS on my hosting provider. And DKIM I will probably have in a year, no kidding, my provider does not plan to implement it in the following months.