How is interest calculated, especially for late crytpo tax filings?

Hi, recently read a post on crypto tax on here and also have a question.

Bought ethereum for bitcoin (taxable event) back in 2018 and didn't know that I had to file taxes for that disposition.

I was told that I needed to pay interest at 6%/year. Would this 6% be calculated based on the supposed capital gain/loss of that ethereum-for-bitcoin transaction or based on the value of ethereum I have now?

Think it's a pretty niche question, but appreciate any insight - thanks for reading.