i think many users of wamp facing this problem, give up, then changed to other service.
if you sure your port already opened, mean this error happens because of your computer/server have 2 webserver installed.
the solution is: stop other service from
Start > Control Panel (switch to Classic view if necessary) > Administrative Tools > Services.
example IIS (from windows)
Right click the IIS Admin and select Properties. Change the Startup type to Manual and click Stop to stop the service. A prompt will appear asking if you want to also stop the SMTP (mail) service and the World Wide Web Publishing Service, click Yes. Click Ok. Locate the Simple Mail Transfer Protocol service and set the Startup type to Manual. Locate the World Wide Web Publishing Service and set the Startup type to Manual.
scroll down the service till 'wampapache' and 'wampmysqld',
double click them, and press 'Start'.
Now, left click to your wamp icon, and "Put Online"
to check your ports open or not: