With the transition to MySQL 8.0, the implementation of INFORMATION_SCHEMA was re-engineered to use views over the transactional data dictionary, leading to column names being returned in uppercase (e.g. TABLE_NAME, TABLE_SCHEMA)
Replace all occurrences of table_name with TABLE_NAME (I counted 7) in nucommon.php and check if the saving error is gone.Some other issues might come up with your versions (PHP deprecations etc). I highly recommend updating nuBuilder to a newer version if possible.