It has been 21 days since my tax return was accepted. My estimate direct deposit date was 02/18/2019. my status is still accepted. I have not received a direct deposit. what happened?

https://www.irs.gov/Refunds

Also, if you have EIC (earned income credit, aka EITC) or ACTC (additional child tax credit), the IRS did not start releasing those refunds until February 15.  That means people with those types of credits may not start receiving their refunds until around February 27.

The IRS says 9 out of 10 filers who efile and choose direct deposit will get their refunds within 21 days.  That still leaves a considerable number of people who do not.   Some returns take longer for additional processing due to specific items in the return.  Some are pulled at random for more thorough investigation.

If you are not subject to the delay due to EIC and ACTC as explained above, and if the IRS "Where's My Refund" tool shows that your Federal return has been processing longer than 21 days, you can phone the IRS, if you wish, to see if you can learn anything about the delay.

IRS:  800-829-1040  hours 7 AM - 7 PM local time Mon-Fri

Listen to each menu before making the selection.
 
First choose your language. Press 1 for English.

• Then do NOT choose the first choice re: "Refund", or it will send you to an automated phone line.
• Instead, press 2 for "personal income tax".
• Then press 1 for "form, tax history, or payment".  
• Then press 3 "for all other questions."
• Then press 2 "for all other questions."  It should then transfer you to an agent, but there is likely to be a long wait.

I usually use a speakerphone so I can work on something else while waiting.