This articles explains the reason why the field properties of a table become locked and unmodifiable:

Why?

The main reason, that the field properties get locked is because you have a form associated with the table open:

Once the form is closed, the field properties will become modifiable.

See also:

• Access, Add Leading Zeros Custom Number Formats
• Access, Convert User Input to Uppercase
• Access, Convert User Input to Lowercase
• Access, Fixed Decimal Places Number Format
• Access Check If Field is Null, Conditional Macro
• First Letter Uppercase Only Access Tables
• First Letter Uppercase Only Access Form

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