You will need to Decode the database before attempting your conversion. I found the following Microsoft article about it:
Overview of Access security (MDB)
Here is an excerpt from it:
The simplest method of protection is to encode the database. Encoding
a database compacts the database file and helps protect it from being
read by a word processor. Encoding a database that employs no security
measures has no effect, because anybody can open the database and gain
full access to all objects in the database. Encoding is particularly
useful when you transmit a database electronically, or when your store
it on floppy disk, tape, or compact disc.
Before you can encode or decode a Microsoft Access database, you must
be either the owner of the database or, if the database employs
security measures, a member of the Admins group of the workgroup
information file that contains the accounts used to help protect the
database. You must also be able to open the database in exclusive
mode, which means you must have Open/Run and Open Exclusive
permissions.
Decoding a database reverses the encoding.
Once you decode it you should be able to run the conversion.