Hello,
LLDP was introduced in VOSS 6.0. You need to upgrade your switch first. It is a good idea to get a newer version anyway as nowadays the current major release is 8. Be sure to check a proper upgrade path. You cannot go from 5 to 8 directly. Validated upgrade paths are VOSS 6.1.x or VOSS 7.x to VOSS 8.0. Also to upgrade to 7.x you need to run 6.1.x first.