To unlock the Teambuilding trophy in FIFA 19, you will have to complete 10 of the Ultimate Team Squad Building Challenges (SBCs).
To find the SBCs, load Ultimate Team >
to Single Player > Squad Building Challenges.
The easiest way to complete 10 SBCs is to complete the challenges from the Basic tab, or Bronze Upgrade challenge from the Upgrades tab. There are more than 10 challenges available in Basic, for the Bronze Upgrade, you can complete it as many times as you like.
Once you have completed and submitted 10 SBC challenges, the trophy will unlock.