The Tax Cuts and Jobs Act ended the individual mandate starting in 2019. So for 2018, you’ll be penalized unless you have minimum essential health coverage or qualify for an exemption from coverage. You can find a list of exemptions in the instructions to Form 8965 (https://www.irs.gov/pub/irs-pdf/i8965.pdf).
A written determination issued to a taxpayer by the IRS that interprets and applies the tax laws to the taxpayer’s specific set of facts. A letter ruling advises the taxpayer regarding the tax treatment that can be expected from the IRS in the circumstances specified by the ruling. It may not be used or cited as precedent by another taxpayer.