The SQL Error: ORA-00904: : invalid identifier is probably being caused because your FOREIGN KEYs are referencing a column that does not exist - check that the column names are spelt correctly and that should solve it (and then your CREATE TABLE statement will work).

CREATE TABLE ArtistMember (
  user_ID INT PRIMARY KEY
);

CREATE TABLE FollowerMember (
  user_ID INT PRIMARY KEY
);

CREATE TABLE pledge (
    pledge_ID            INT CONSTRAINT  XPKPledge PRIMARY KEY,
    pledge_endDate       DATE NULL,
    pledge_startDate     DATE NULL,
    pledge_amount        DECIMAL(9,2) NULL  CONSTRAINT  Currency_1322638346 CHECK (pledge_amount >= 0),
    artist_userID        INT NOT NULL CONSTRAINT gets REFERENCES ArtistMember (user_ID),
    follower_userID      INT NOT NULL CONSTRAINT makes REFERENCES FollowerMember (user_ID)
);

INSERT INTO ArtistMember VALUES ( 85275 );
INSERT INTO FollowerMember VALUES( 88128 );
INSERT INTO pledge VALUES(
  559,
  NULL,              -- Use NULL and not 'NULL'
  DATE '2016-02-01', -- Use a Date literal and not a string literal
  3850,
  85275,
  88128
);

If you just use the string in '1-FEB-2016' then Oracle will implicitly try to convert the string literal using the TO_DATE() function with the NLS_DATE_FORMAT session parameter as the format mask. If they match then it will work but this is a client variable so can be changed and then the query will break without the code having changed (and be a pain to debug). The simple answer is to ensure that you compare date value by either using TO_DATE() and specifying the format mask (as per the query above) or to use an ANSI date literal DATE '2016-02-01' (which is independent of the NLS settings).

They are different. When you enclose something in single quotes, it becomes a string -- this is true of all keywords in SQL, including NULL.

When you use just NULL, it is the SQL keyword, and it is compatible with all types.

So, these are quite different:

insert into t (col1, col2)
    values ('2021-05-25 11:58:41.000', NULL);

and:

insert into t (col1, col2)
    values ('2021-05-25 11:58:41.000', 'NULL');

'NULL' is a string and only compatible with a string type. You should get a type-conversion error if the column is not a string.

The one caveat is if you are storing the values in a text file for loading into the database. In such a file, strings may or may not be delimited with single quotes, depending on the structure of the file.