I am having the exact same problem with a my DJI Mavic Pro 2 and my Samsung A22 or Huawei P20 Pro. All pre-flight checks are passed, the green Start Flight button is flashing ready to go, but it does not respond when I touch it. I have used the P20 Pro and this drone for more than 18 months hassle free, and then this started.
On a side note, I also find that the flight plans always default to Mavic Enterprise instead of my usual drone model, which is not a problem I have had previously too.
Other users are also reporting this issue: Touchscreen buttons/icons not responding