The standard rules are:
Food, Water, and Defense meters on the HUD on the left to 100% full.
Settlement Happiness above 70.
Plot has power.
Enough resources for them to be able to afford their upgrade.
At least 3 ingame days to have passed since they were built/since the last upgrade.
If you do not meet any of those requirements, the "HOLO HUD" over each plot will show a red icon in the bottom row that tells you what they are missing.
More info on the HUD can be found here:
https://wiki.simsettlements2.com/gameplay/basics/HUD