Am I able to send additional documentation to state after submitting 1040 in TT? I was shorted a credit from state, possibly due to not attaching K-12 tuition receipt.

You cannot change or add anything on the return that you just e-filed, nor can you stop it.  It is too late, just like when you put an envelope in a US mailbox on the corner.  The IRS does not allow you to take it back.

If you left out a W-2, a 1099G, or a dependent, or a 1099 etc…DO NOT change your return while it is “pending.”  The changes will go nowhere.

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 make the necessary changes to your tax return and re-submit your return. 

 If the IRS accepts your return, however, then you have to wait longer until it has been fully processed and you have received your refund.  THEN you can prepare an amended tax return and e-file or mail  it in. You have to be able to work from that return exactly the way it was when it was e-filed originally.  You will need to use a form called a 1040X.

  Meanwhile, DO NOT go in and start changing anything on your return in the system, or you will make a mess for yourself.  Sit tight and wait until you see what the IRS does with the return you just e-filed

The Form 1040X you need will not be available until late February.

To amend only a state return:

https://ttlc.intuit.com/questions/2824532-i-need-to-amend-my-state-return

https://ttlc.intuit.com/questions/3827521-can-i-efile-an-amended-state-return