The achievement says "Clear the 1st through the 200th floors of the Palace of the Dead solo".

You can see from the previous achievements before it that starting from the 50th floor does not count.

"Clear the 1st through the 100th floors of the Palace of the Dead solo and record a score on floor 101 or below".

You most likely didn't get that achievement or the one below it.

"Clear the 1st through the 50th floors of the Palace of the Dead solo and record a score on floor 51 or below".