How to make futuresHelpers in Quantlib work with monday settlement day not IMM?

Eurodollars futures contracts stop trading on 2 business days (usually a Monday) before the third Wednesday of the expiry month.

BUT that is not the date that the reference 3 month interest rate, 3M USD Libor, starts. That date is actually the third Wednesday itself, which is the IMM date (see contract specification here)

So in your example, liborStartDate (which is "d", the second argument to FuturesRatesHelper() in your example) should be set to 16 Sep 2020, not 14 Sep 2020. This is why you get an error.

IMM dates are actually Wednesdays and not Mondays...

There are several methods to work with IMM dates in the ql.IMM class.

To check if dates are IMM dates, use ql.IMM.isIMMdate(date, mainCycle=True)

In your example:

futures = { ql.Date(14,9,2020): 99.700, ql.Date(14,12,2020): 99.67, ql.Date(15,3,2021): 99.76 }
list(map(ql.IMM.isIMMdate, futures.keys()))

[False, False, False]

You can also check what is the next IMM date:

list(map(ql.IMM.nextDate, futures.keys()))

[Date(16,9,2020), Date(16,12,2020), Date(17,3,2021)]