Hi thanks for your question.
You’re planning to conceive and you’re on duphaston
from 16th day of your cycle.
You have forgotten to take day 18th tablet.
Take it as soon as you remember, but before the next due tablet.
If you remember by the time next tablet is due, you can omit it, and take next tablet due.
Since you’re not taking it for contraceptive
purpose, missed tablet will not cause any problem
Continue your course as usual.
Hope i answered your question.