Step 1)¬†Is the button greyed out? Check for a timer. Some courses will have the¬†mark complete¬†button disabled until the lesson timer runs out. Once the timer has elapsed, the mark complete button should no longer be greyed out and should allow you to press it.
I have tried the above but the mark complete button is still greyed out.¬†In this case, please contact support at¬†[email protected]