16 so that we can preach the gospel in the regions beyond you. For we do not want to boast about work already done in someone else’s territory.

Read 2 Corinthians 10:16 Using Other Translations

To preach the gospel in the regions beyond you, and not to boast in another man's line of things made ready to our hand.
so that we may preach the gospel in lands beyond you, without boasting of work already done in another's area of influence.
Then we will be able to go and preach the Good News in other places far beyond you, where no one else is working. Then there will be no question of our boasting about work done in someone else’s territory.