You can only build or upgrade one building at a time.
The time remaining until an upgrade is complete is shown in the Settlement Overview on the left of the Settlement screen. If you wish to have two buildings or upgrades simultaneously you can hire Extra Labor.
This will give you two Construction slots. You can also avoid the wait and complete building and upgrades instantly by using Biocaps.
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Game Guide for State of Survival: Survive the Zombie Apocalypse