Trying to use commands to add potion effect to entitys, but they dont get the effect.

I have made a sword and i want it to be able to "stop time" by right clicking. To do this, i am just applying speed 255 to all entitys. However, this command simply wont work. I have tried adding a "give @s dirt" command and that worked fine, i just cannot give entitys potion effects. Any idea on what is happening? ( i tried to upload a picture but i dont know how to get the url, i will try to post it in the comments)