Steps to Reproduce:
Type the following command in chat `/attribute @s minecraft:generic.flying_speed base set 0.2`

Observed Results:
It will say "Entity {your name here} has no attribute Flying Speed". This will happen even if the command is run in the air.

Expected Results:
The command would allow you to change how fast you (or any other player) moves when flying in creative mode, spectator mode, or with elytra