On the control center, I can’t turn my data off or on. How do I fix this?

Try this:

Go to Settings->General->Restrictions->(Allow Changes)Cellular Data, and make sure its not turned off.

If it works from Settings, then it may just be a bug.

Try force restarting the iPhone. (No data will be lost)

• On an iPhone X, iPhone 8, or iPhone 8 Plus: Press and quickly release the Volume Up button. Press and quickly release the Volume Down button. Then, press and hold the Side button until you see the Apple logo.
• On an iPhone 7 or iPhone 7 Plus: Press and hold both the Side and Volume Down buttons for at least 10 seconds, until you see the Apple logo.
• On an iPhone 6s and earlier, iPad, or iPod touch: Press and hold both the Home and the Top (or Side) buttons for at least 10 seconds, until you see the Apple logo.

Long press the the group icon (Airplane, Mobile/Cellular, Wi-Fi & Bluetooth, top left in Controi Centre) to switch

Mobile/Cellular Data OFF/ON.

Can you turn it on/off in Settings->Cellular?

If you cannot, do you have sim card installed in the iPhone? Does it have a data plan attached to it?