In most cases, you can't deduct your share of health coverage costs for a group plan offered through your employer.
The reason? Most premiums are paid with pre-tax dollars, which means they are deducted from your wages before taxes are applied. Deducting them again as a medical expense would be "double-dipping."
You can only deduct the premiums if your employer included them in Box 1 (Gross Wages) of your W-2. However, this is highly unusual and contrary to the standard practice.
Similarly, HSA and MSA contributions that come out of your paycheck aren't deductible either, as these contributions are funded with pre-tax dollars.