The message is misleading. The "enter" script is used for a room when the player enters a room, so cannot also be used for a verb. You could add your own command.
This came up about a year ago when someone tried to add an "enter" verb, and it caused havoc, so the warning got added in 5.7.1.
UPDATE
Adding this to the game allows me to just use an "enter" verb:
<verb name="enter_verb">
<pattern>enter #object#</pattern>
<defaultexpression>"You can't enter " + object.article + "."</defaultexpression>
<property>enter_verb</property>
</verb>
That's much easier than fooling around with a command, especially if you have more than one scenario when the player could enter something.