Should an entire $9,426 long-term capital gain carryover from 2015 offset $21,757 in capital gains in 2016? If not, how can I correct this error on the 2106 return?

[fanfare]

It is not an error. that's how it works.

[Critter · Accepted solution]

I hope you mean you had a 2015 long term capital LOSS carryover since gains cannot be carried over ... and if you did then yes, the loss is netted against the 2016 cap  gains.  

If you failed to enter the LOSS carryover in the 2016 program then you can amend the return to correct the error ...

https://ttlc.intuit.com/replies/5114064