After I appealed, IRS agreed to return the extra tax they collected. But later they closed my case without refund me

In 2015, I got a letter from IRS telling me that my 2014 tax-return calculation is not correct and asking me to pay extra tax.  I followed their order and I also sent IRS my appeal letter that my calculation was correct.  After several round of communications with IRS, They finally sent me a letter in Nov 2019, telling me that they recalculated my 2014 tax-return and would return me (by check) the extra tax I paid.   Half year later, I sent IRS inquiry letter asking when I could receive the check from them.  I received a letter from IRS in Dec 2020 telling me to wait as they are still work on it.   One week ago I received a new letter from IRS telling me that they would not handle my case anymore since they don’t allow any claim for tax-return issue occurred seven years ago.   

Initially I planned to go to IRS local office to ask for help, but it need appointment first and I could never get through the IRS 800 phone to make appointment.   

How I can get my money back?   Thank you very much.