Assuming you are preparing your 2019 Form AR2210, you are being asked to enter your 2018 Net Tax from your Arkansas Form AR1000F if you were a resident, or your AR1000NR if you were a non-resident.
If you filed a 2018 Form AR1000F you would look at Line 38 for the prior Net Tax. If you filed a 2018 Form AR1000NR, you would look at Line 38D to locate the prior Net Tax.
If you did not file a 2018 Form AR1000 and this is the first year you will file in Arkansas, then simply enter 0 for the prior year Net Tax.