Features at the correct levels
The core design of D&D 5th edition ensures that there is a balance of features at each level for a player.
What this means is that each core class has gaps in their features - these gaps are where the subclasses for that class provide their features.
D&D Beyond adheres to this philosophy, so it is important that you do when you're designing your subclasses, if you want to be able to publish the homebrew for public use!
You may, of course, add whatever features you like to your subclasses for personal play and sharing privately via a campaign - this rule is for published homebrew.
The table below shows what levels each class has gaps in the features, where you should place features for your subclass:
As an example, let's take a look at the Fighter class table, where you can see I have marked the entries for Martial Archetype and Martial Archetype Feature. These match the table above. Any Fighter subclass designed will need to have features at these levels to be valid for publishing, further if the subclass has features outside of these levels, that will also disallow it from publishing.
This section can cause some confusion.
For classes with no innate spellcasting (Fighter/Monk/Rogue) - make sure to set this to NO unless you are adding spellcasting through the subclass, at which point you should set this to YES. If you leave it blank, it may cause issues.
For classes that do have innate spellcasting (all the other ones) - make sure to set this to NO or leave it blank, as you aren't adding spellcasting.