Hi Brenda!
It's quite self-explaining, There are no units available for your tape mount 
request.
Because the 'wait for unit' bit in the ADSM dynamic allocation routine is not 
set, this message is issued. In our situation we do not use tape directly (we 
create disk datasets which are migrated to tape later on by SAMS:DISK) I cannot 
help you with solving this problem, but I guess you can somewhere modify the 
dynamic allocation routines ADSM uses for tape dataset allocations.
Or maybe it works as designed?
Perhapse someone else can clarify this?
Kindest regards,
Eric van Loon