I found many possibilities how to set the parameter. Finally I has a correct script:
LET SetDB=NULL(); // delete old value
IF (Len(Trim(DB))=0 or IsNull(DB)) THEN
LET SetDB = 'DB0';
LET SetDB = $(DB);
LET DB=NULL(); //delete old value
VERY IMPORTANT! I have to add new parameter DB into Settings - Variable Overview! My parameter did'nt exist before.