“ORA-01109: database not open” when trying to open a database

Everything I can find about ORA-01109 says "run alter pluggable database XXX open;", but I’m getting ORA-01109 when running that command!

select name, open_mode from v$ containers;

NAME         OPEN_MODE  ------------ ---------- CDB$  ROOT     MOUNTED    PDB$  SEED     MOUNTED    XEPDB1       MOUNTED 

alter pluggable database cdb$ root open;

Error starting at line : 2 in command - alter pluggable database cdb$  root open Error report - ORA-01109: database not open 01109. 00000 -  "database not open" *Cause:    A command was attempted that requires the database to be open. *Action:   Open the database and try the command again