When attempting to create a new Monitor, I noticed that the Check Periods field is blank. If I go into an existing Monitor, it is not blank. Why is this happening?
You are at the maximum number of monitors for your current plan. To find out what your Monitor usage is, click on Monitoring -> Monitors. Then scroll down to the Monitor usage overview section. This will tell you how many Monitors you have used and how many are remaining. You should see 0 (zero) Remaining in this situation.