Sorry about the confusion.
Initially I thought I did not file form 8606 in those years (2015-2018) but I was able to discover that I've been filing form 8606 all along in my tax returns. I discovered the forms in my previous tax returns in my turbotax account.
Then for the 2019 form 8606, which I did in my most recent tax return in July 2020, it was not accurately filled by turbotax software for some unknown reasons
However,the 8606 forms for 2015-2018 were not also accurately filled.
Therefore I'll be amending my tax returns
