It's likely that your accountant is wrong, but there's a small chance that your accountant is correct. The amount of your net earnings available to contribute to a traditional IRA is reduced by the amount of your deductible contributions made to your solo 401(k). If your net earnings are low enough, it's possible to contribute all of your net earnings to a solo 401(k), leaving none available to contribute to a traditional IRA.
However, with net earnings in excess of $18,000 ($24,000 if age 50 or older in 2016), there will be *some* net earnings left to contribute to a traditional IRA, perhaps enough to make the maximum traditional IRA contribution. Since your solo 401(k) contributions make you covered by a workplace retirement plan, whether or not that traditional IRA contribution will be deductible will be determined by your filing status and modified AGI.
