Hi Ramon,
It’s probably the version since the 3550 does support DHCP snooping. It’s best to get one of the “IP services” images. This is what you should do:
Go to Cisco.com -> Support.
Select the 3550.
Select Download Software.
Select one of the EMI versions.
Select Downloads.
Select IOS Software.
The latest “IP services” version is “c3550-ipservices-mz.122-44.SE6.bin” but an older version would also work.
Here’s the direct link if you want:
https://software.cisco.com/download/release.html?mdfid=275935148&softwareid=280805680&os=&release=12.2.44-SE6&relind=AVAILABLE&rellifecycle=&reltype=latest&i=!pp