Decent plugin, does what it says. Though there is some problems.

The naming of the main class: You should name classes capitalizing each word, "RandomEffects" over "Randomeffects"
Language: Potentially have more languages for the logger, as it's just Russian (that's Russian afaik)
Everything in the main class: You should split each thing into a different class for better organization
Variants: You don't need the "minecraft:" part
Commands: You are running server commands for effects rather than taking the target and adding an effect that way
"pillow": You have this pillow static int that is there which is very unnecessary and the usage is not very smart and is very laggy. You are sleeping to pass time in the loop. Rather than doing this, you should actually see how the functions work, along with you forgetting a comma at the end of your scheduled task. you need to put a delay for the repeating task as a time