Skipping the CRA Access Code and leaving the field blank worked for me. (Just another failure of the government in my opinion. There own Access Codes not communicating).

It turns out that the NETFILE access code is now changing every year. The access code I used up until 2021 is not valid anymore. On my NOA from 2021 there is a new Access Code. I used the new code and I was able to file. The access code that appears now on the 2022 NOA is a new code as well. 
It would be nice to have this clearly explained somewhere so we don't try to submit blindly until we get looked out. I hope others find this helpful.

It turns out that the NETFILE access code is now changing every year. The access code I used up until 2021 is not valid anymore. On my NOA from 2021 there is a new Access Code. I used the new code and I was able to file. The access code that appears now on the 2022 NOA is a new code as well. 
It would be nice to have this clearly explained somewhere so we don't try to submit blindly until we get looked out. I hope others find this helpful.

System wont accept netfile code.

Dont have notice of assessment to locate it

@fleetwoodesltuto the code is not mandatory, you can try filling your return without the code.

You can get back to the page with your NETFILE Access code by clicking on “My Info”, then “[Your Name]’s Personal Information”. Then scroll to the middle of the page to find the code entry box.

I had the same issue for all three on my family returns. We verified the Netfile # on each NOA and it matched. Turbo Tax advisor said to enter without but it makes it difficult to communicate with CRA on current taxes. I have used same Netfile codes with out error for years. I feel like this is a bug in the system!  Disappointed with TurboTax support although they were able to remote in I was still no further ahead @Turbotax Hope you have better luck!

I tried this and verified on my NOAs. This did not fix the issue, Same warning came up that could not be cleared every time before I received the Code 21. There is a bug in the coding. Very glitchy this year and I have used online for years.

Thanks, that worked for me.

ready to file but get error code 21 from CRA

does the access code from my notice of assessment change every year or remain the same, 

I could not get to the page you described to delete the code.

What happens when you are locked out??