Looks like your icing got broken when you duplicated faces off the donut and before you applied the solidify and subdivision modifiers. You either missed some faces when selecting the top of the donut to duplicate for icing, or split some vertices afterwards. You’ll have to go back a few steps since you can’t unapply the modifiers and trying to stitch them back together will be far more painful and getting it smooth will between the icing pieces.
If you hide your icing then go into edit mode on your donut, you’ll see you still have duplicated faces from making the icing (looks like you duplicated it a couple times) but they are also patchy. Reconnecting the split vertices leaves artifacts so you need to go back further.
In the file I’m providing, I deleted the extra duplicates on the donut, and reselected the same faces (without islands or missing pieces) and duplicated it as one piece to make your new icing, then added a new solidify modifier. I also renamed the materials for clarity, but that’s not relevant to the process. You can either refer to this file and follow the same steps, or use it as is to jump start the process. You’ll need to apply the modifiers to the icing and resculpt the icing (or was that proportional editing for the icing?), make a new weight paint for your sprinkles and copy over your particle settings.
Get the modified file here
Useful keys for dealing with this stuff in the future. The [L] key selects connected faces (useful for finding disconnected bits or selecting by connected faces. [M] > Merge by distance can usually reconnect split and duplicated vertices if they’re still on top of eachother.