How to inform user they canot remove an element?

Question

So I am working on a software for repair and maintenance companies right now. We are going to have a customer database.

Once you add a customer to the database, you can only remove them if there are no maintenance requests associated with them, past or future (requirement for traceability, especially for quote documents, etc.).

How to inform the user?

1. should I keep the "Remove" button in the menu, but disable it and display a toast/modal/tooltip with the reason why it cannot be removed?
2. do I remove the "delete" option from the menu at all?
3. any other idea?

Answer 1

The best option is to explain to the user why he can't be removed. A tooltip or a modal with the explanation will explain the reason. Keep the button visible and disabled, and let the user knows that options do exist but its not available due to some reason.

Do not hide important elements of your interface that are always visible without informing the user.

Can you imagine, you just get home and your recycle bin is gone and nobody explains why? It's better to find the recycle bin locked than to dont find it.