I have an issue in a call setting I am trying to activate the call waiting and I can’t what I can do

Call waiting is a carrier feature, you have to ask them.

See here:

Call forwarding, call waiting, and caller ID

Set up call forwarding, call waiting, or caller ID. (GSM) Go to Settings > Phone.

• Call Forwarding: "a phone icon" appears in the status bar when call forwarding is on. You must be in range of the cellular network when you set iPhone to forward calls, or calls won’t be forwarded.
• Call Waiting: If you’re on a call and call waiting is turned off, incoming calls go directly to voicemail.
• Caller ID: For FaceTime calls, your phone number is displayed even if caller ID is turned off.

For CDMA accounts, contact your carrier for information about enabling and using these features.

For more information, see the Apple Support article Call forwarding, call waiting, and other call features on iPhone.

islam186,

Go to Settings > Phone > Call Waiting. Can you select it? Perhaps that feature is not supported by your carrier.