To give you more details why your power symbols did not work:
In kicad power symbols are implemented a bit hacky. They use invisible power input pins which makes them into a global label with the net name taken from the pin name. In addition they force you to declare where power comes from
(error type 3 -> solution: there must be a power output pin somewhere on the net. Either the symbol for your power supply has a power output pin or you need to use the pwr_flag symbol. See link posted by @John_Pateman for more details.)
A bit more detail about how to create personal power symbols is given here: Tutorial: How to make a symbol (KiCad v5.1.x)