Did you follow the instructions on this post originally? If so, that’s where you set the password for your database. Or did you follow some other guide? It seems like you didn’t grant the openhab user access to the openhab_db. Probably you did create an admin password?
$ influx
Connected to http://localhost:8086 version 0.13
InfluxDB shell version: 0.13
> CREATE DATABASE openhab_db
> CREATE USER admin WITH PASSWORD 'SuperSecretPassword123+' WITH ALL PRIVILEGES
> CREATE USER openhab WITH PASSWORD 'AnotherSuperbPassword456-'
> CREATE USER grafana WITH PASSWORD 'PleaseLetMeRead789?'
> GRANT ALL ON openhab_db TO openhab
> GRANT READ ON openhab_db TO grafana
> exit
In case you don’t remember what you did, you can stop influxdb, turn off authentication, start it up and enable it with a new password. See this for more information.