Gen Z Bees have a Mission Cap which determines how many missions they are able to go on. 

You can add more Missions to a Bee's Mission Cap by upgrading any stat. Each upgrade will add 10 additional Missions to that Bee's Mission Cap. Visit the Bee Lab to upgrade your Bees.

You can keep adding to your Mission Cap until your Gen Z Bee reaches his Mission Span. Mission Span is the amount of days the Bee can perform Missions. Mission Span cannot be extended.

Once your Bee reaches his Mission Span, you can convert him to shards. Visit the My Bees section to convert your Bee.

NOTE: Retired Gen Z Bees can still be used to do Quests and PvP

For more information, see this link