It's possible that you actually qualify for this credit, but don't need it.
The Joint Filing Credit is a non-refundable credit, meaning that it can only offset any tax that you might owe. To start your taxable income computation, the Ohio return starts with your federal return's adjusted gross income, then makes state adjustments and exemptions, among other computations, so it's possible you have no Ohio tax liability. If you don't have any state tax liability on Form IT 1040, line 8c, there's no tax to offset.
You must also have a certain amount of "qualifying income" for the credit to apply. According to the Instructions for Form IT 1040, also available in the link above:
To qualify for this credit, you and your spouse must each have at least $500 of qualifying income and jointly file your return. "Qualifying income" is any amount included in Ohio adjusted gross income, other than the following:
●Interest;
●Dividends and distributions;
●Capital gains; AND
●Rents and royalties.
Amounts deducted on the Ohio Schedule of Adjustments are not included in Ohio adjusted gross income, and thus are not "qualifying income." Examples of such amounts include business income, state and local tax refunds, Social Security and railroad retirement benefits, and certain military compensation and retirement benefits.
Another possibility is that you have other credits that have offset your tax liability, before the Joint Filing Credit is applied. If line 11, Tax less credits, on your Ohio Schedule of Credits (also found in the link above), is zero, then the Joint Filing Credit wouldn't be needed.
If none of these appear to be the issue, please re-post with any additional information.
