Hi there deeneals,
If you are unable to change the skin tone of an emoji by touching and holding it in the emoji keyboard. First follow the steps here to restart your iPhone:
1. Press and hold the side button until the power-off slider appears.

2. Drag the slider, then wait 30 seconds for your device to turn off. If your device is frozen or unresponsive, force restart your device.
3. To turn your device back on, press and hold the side button until you see the Apple logo.
This is found here: Restart your iPhone
Also ensure your iPhone is fully updated:
1. Back up your device using iCloud or your computer.
2. Plug your device into power and connect to the internet with Wi-Fi.
3. Go to Settings > General, then tap Software Update.
4. If you see two software update options available, choose the one that you want to install.
These steps are from this article: Update your iPhone, iPad, or iPod touch
If this issue continues after restarting and updating your device, then we would recommend reaching out to Apple Support to get the best assistance going forward.
Get Support
Take care!