it's nothing to do with the balance in your IRA that is a common situation. If you have an IRA balance it will affect the taxability of the backdoor Roth as the Roth conversion is considered taken in a pro-rata manner from your pre-tax balance and basis, so some part of the backdoor Roth will be taxable, and the remainder of your contribution will rollover into your basis for future years until you bring the Trad IRA balance to $0.
If you are getting an over-contribution message then my guess is you don't have enough "earned" income (W2 etc). IRS sets a contribution limit, but you also can't make IRA contributions (to Trad or Roth) above your earned income.
For reference here are the steps
https://ttlc.intuit.com/turbotax-support/en-us/help-article/retirement-benefits/enter-backdoor-roth-...
