CodeIgniter Core files errors

Hello,

I' m getting these errors within CodeIgniter Core files. Any Ideas on how to fix them?

error 1:
Message: Trying to access array offset on value of type null
Filename: core/Config.php
Line Number: 67
error 2:
Message: Undefined array key "subclass_prefix"
Filename: core/CodeIgniter.php
Line Number: 237
error 3:
The configuration file database.php does not exist.