It seems another Apache process is using port 80. You can find out which one with the following command:
sudo netstat -tulpn | grep 80
That will output the process and the PID:
tcp6 0 0 :::80 :::* LISTEN 2227/httpd.bin
Where the PID would be 2227 in this example.
Then you can kill the process with the following command:
sudo kill -9 PID
Changing PID with the actual pid number.
After that it should be possible to start the stack's Apache.
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