There are strict deadlines for appealing a denial of unemployment benefits in Missouri. You have 30 days to appeal from the date your benefits are denied. Look at the "determination" (written decision) that you got from the Division of Employment Security. There will be a date on that page that gives you the deadline to appeal. If that date has passed, the deadline to appeal has run out.
Here's a link to a toolkit about the unemployment appeals process that you can use to learn about the deadlines, in case you need to know in the future.
If you still think you can meet the deadline to appeal or give a good reason why you missed the deadline, you can submit an appeal on UInteract or using the form on DES's website.