Is there a maximum income level for a teacher who wants to get the tuition tax credit for classes taken to renew state teaching certificate?

I took 2 graduate classes in 2018 as I needed to renew my MD state teaching certificate.  I tried to deduct the cost of these classes on my tax return but Tubo Tax stated that I was not eligible for an education deduction or credit possibly due to the following:  my income exceeds the maximum limit or there were no net qualified education expenses.  How can I find out exactly why I cannot get this tax credit? I thought classes that were used for a professional certification/continuing education as a teacher qualified for a tax deduction in Maryland.