@DerEchte @mp3 not sure if you managed to fix it, but this is what I did.
I had the same issue right out of the box, so I tinkered a bit with plikd.cfg. I created a database with mariadb, then edited the Driver value under MetadataBackendConfig section in plikd.cfg from sqlite3 to mysql. I also changed ConnectionString value, according to this here, from /data/plik.db to user:pass@tcp(db_IP:port)/dbname?charset=utf8mb4&parseTime=True&loc=Local. So now it looks like this:
[MetadataBackendConfig]
Driver = "mysql"
ConnectionString = "plik_user:plik_password@tcp(serverIP:3306)/plik_db_name?charset=utf8mb4&parseTime=True&loc=Local"
With real values, instead of placeholder ones, of course.
However, I stumbled upon another issue once I got Plik up and running. Whenever I tried to upload a file, I got "error 500: unable to save file : unable to create upload directory". I went to check permissions of ./plik/data, and it turns out even though I followed instructions on creating the dir before installing the Plik image, /data didn't have the right permissions. I changed it to 777 and that fixed the issue.
In hindsight, incorrect /data permissions were probably what blocked Plik from creating its database, hence the "failed to initialize database" error.