Forgot to include cashed savings bonds in 2019 tax filing, what can I do about this?

You should file an amended return.

You cannot change or add anything on the return that you e-filed.  It is too late, just like when you put an envelope in a US mailbox

Now you have to wait until the IRS either rejects or accepts your return. If your return is rejected, you will be able to go into your account and add or change it and resend it.

If the IRS accepts your return, then you have to wait until it has been fully processed and you have received your refund or paid the tax due. THEN you can prepare an amended tax return 1040X and mail it in. You cannot e-file an amended return.  Meanwhile, DO NOT change anything on your return until you start the amended return. If you did change or add the missing info then take it out and put your return back to the way it was when you first filed it.

You can check your efiling status here,
https://turbotax.intuit.com/tax-tools/efile-status-lookup/

How to amend an Online return
https://ttlc.intuit.com/community/amending/help/how-do-i-amend-my-turbotax-online-return/00/27577

There federal return has been accepted by the IRS and I'm not seeing an option to amend as shown in the video in that help article. Does it take time for the option to become available after the refund has been accepted?

I don't know.  But you need to wait for the IRS to finish fully processing your original return.  And they are not processing mailed returns now anyway so there's no hurry.  You have 3 years to amend.