SQL doesn't have a "--" operator - instead "--" indicates the start of a comment.
What you need is:
SET @Finalvar = @Finalvar - 1;
But even with that fixed, the code won;t work: you will get a conversion error on yoru CAST:
set @coun+= cast(@bck as tinyint);
Because the value in
is not a number:
declare @bck varchar(5)='bck'+@finalvar