"An object named "MSSql$SqlExpress" cannot be found"

"An object named "MSSql$SqlExpress" cannot be found"
I have previously added this user to the data folder using Properties->Security-> etc... and the user showed in the list and it worked. Today I tried to add the same user into another folder and it gives the error message. I cannot find the user MSSQL$SQLExpress anymore, nor even in the folder where it is listed. I'm using SSMS 2014 and SQLExpress and I'm able to access the databases and read/write data