It looks like the SDSF loadlib is not in your linklist. Maybe it's allocated dynamically when SDSF in invoked.
From anywhere in ISPF, do DDLIST on command line, then LP to list LPA+LINKLIST, type MEM ISFCALLS and YES to the popup. That will tell you if ISFCALLS is findable. Where I am, it's in SYS1.ISF.SISFLOAD.
If it's not found, go into SDSF and try the same. This time I would expect it to be found under a dynamic ddname.
Try googling "sdsf rexx isfcalls not found" and you will find that other people have had the same issue. I think SDSF REXX was first introduced with zOS 1.9 so it should be available at your site, but I guess something is missing in the configuration. Talk to your zOS Administrators, they should know.
Hope you get this to work. SDSF REXX is a very useful and powerful tool !