Table &1 is extendable without limits.

What causes this issue?

The table is defined as extendable without restrictions.
Users can append STRING fields to the table, but STRING fields are not
supported by the view maintenance, so any fields of type STRING which
are appended to the table cannot be displayed and maintained in this
maintenance view.

System Response

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

How to fix this error?

Set your table to 'Extendable and Character or Numeric', as follows:
1. Go to the ABAP Dictionary (SE11) initial screen.
2. Enter table name -> Change
3. Menu Extras -> Extension Category...
4. Choose 'Extendable and Character or Numeric'. Copy and activate.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.