There is no error to correct "per-se". You're just not taking the deduction for the PMI paid, is all. For most, amending to take that deduction does not change their tax liability anyway.
Basically, if the total of all of your itemized deductions on SCH A do not exceed your standard deduction "after" adding the PMI deduction to SCH A, then amending will not change your tax liability in any way. So it would be a waste of time to amend it.
Not amending will have absolutely no effect on your 2018 tax return next year.