SQL print a space between concat statements

You don't mention what flavor of SQL you're using, but depending, you may need to convert the values to strings first. For SQLSever...

(Cast(NBR as varchar(20)) + ' ' + Cast(ACCT_NBR as varchar(20))) as acct,

Though you didn't mention the database, try

MySQL

concat(NBR,' ',ACCT_NBR) as acct

SQL Server

CAST(NBR AS VARCHAR)+' '+CAST(ACCT_NBR AS VARCHAR) as acct