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
Linked issues
Comments

Invalid, as the message tells you, players do not have a flying speed attribute. For more info, see MC-180199.