Its not an error. It is an informational message telling you that you do not have exclusive rights to the database to make and save design changes.
In Access, the first person to open a database has the rights to edit it. Once they close it, they release those rights.
So if someone else already has it open, you will get that message.
Or, if someone doesn't close down the database properly so that the database lock is not released, you will also see that.
Take a look at the folder where the database resides in Windows Explorer. Whenever the database is open, you will see two files with the database name, the second with a file extension starting with the letter "l". This is the database lock. Once everyone is out of the database, that file will disappear. If everyone is out, and it still appears, try deleting the locking file.