Read-Only field in user Profile (Here's the current code, what Do i do now?)

Hi I’ve created a custom field in my functions.php to the user profile.

This field serve as a “registration” number and must be view only by the user in his profile and read-only except for an admin who is able to edit it.

Here is a paste of the code i’m using. Can someone paste me a corrected version of what I need and explain how it work please?

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