Hello Bernard
Unfortunately, Packet Tracer doesn’t support all of the commands and features available on real Cisco devices. This is one example of unsupported commands. Alternatively, you can use the GNS3 emulator which includes all of the commands available in a particular IOS image, or a choice of several other emulators including Cisco CML. More info on these can be found at the following lesson. The lesson may refer to CCIE, but the same applies for all Cisco certifications:
I hope this has been helpful!
Laz