"this computer has been scheduled to go to sleep" when none is set
I had this issue as well. I found two potential answers:

This one suggests resetting PRAM and the SMC:

https://discussions.apple.com/thread/7702633

However, the second one mentions iDrive (as did a previous answer here). I did, in fact, switch to iDrive about a month ago, and believe the dialog box began appearing after installing it:

https://discussions.apple.com/thread/8304128?page=2

The above thread refers to a response from iDrive support which suggests un-checking the "wake from sleep" setting (it is apparently checked by default; mine was). For me, the issue was completely resolved by making this change; it has been 4 months since I changed the setting with zero recurrence. Make sure you don't miss the "do you want to save changes" dialog after changing it; iDrive has a way of making macOS feel like Windows.