When attempting to create a homebrew spell I'm trying to add an effect that sets an affected creature's speed to 0 (similar to Hypnotic Pattern). However, when I try to do so, I get an error saying the Fixed Value field must be at least 1. However, if I make a copy of Hypnotic Pattern, I see that the Fixed Value is set to 0.

I had noticed that too.

The spell doesn't need that modifier anyway (not even sure why hypnotic patter has it). Spell effects besides damage and conditions are 100% description. Modifiers are for automatically implementing a static change on your character sheet.

So far.... After all of the redesign is done that might change. Especially if they are trying to integrate everything into a VTT. Don’t look at how it has been, look at what it will need to be sooner or later because of what it will become.

Quote from IamSposta >>

So far.... After all of the redesign is done that might change. Especially if they are trying to integrate everything into a VTT. Don’t look at how it has been, look at what it will need to be sooner or later because of what it will become.

 Maybe... But VTT is still pretty far away. So far that the homebrew tools will almost certainly work differently by then.