How can I disconnect my water line from the toilet?
If the line is made from solid copper (often chrome plated), it sometimes has a bit of tension in it, making it appear to be stuck to the fixture.    Try gently pushing it in different directions while pulling down.

If the line is a flex line, it may be stuck with a bit of mineral buildup; try gently wiggling it back and forth.

If you're not comfortable with the amount of force needed, it may be too much.   Consider calling a pro rather than breaking something.