It doesn't recognize "practise" as a correct spelling.
You may remember from your elementary school days: practice with a C is a noun, and practise with an S is a verb.
So how come Spell Check can't figure out the difference between the two?
Anyhow, if you are writing something in Word and you use the word "practise," make sure that Spell Check doesn't automatically change the word to "practice" if it's not a noun.
I need to start practising my singing every day. (verb)
My singing practice has been a little lax, lately. (noun)