Samantha,
Lets' review the invoice side first... You create an AP Invoice for a EUR Vendor for 100 Euros and LCY of 200. On your Exchange Rates for Euro you entered a 2 under the Relational Exch. Rate Amount field. Now you are ready to pay this 100 Euro, 200 LCY Invoice. Today is 3/17/21 so the Invoice Exchange Rate from 8/4/20 will not be correct since Exchange Rates change daily. The Payment Exchange Rate will be different and therefore generate an Exchange Rate Gain or (Loss). Also, per GAAP, at the end of each month you should be running the Adjust Exchange Rate to update your Balance Sheet and Income Statement entries. This keeps the true amount of the Foreign Currency Invoice close to what the payment exchange rate could be.
What Exchange Rate have you entered for the date you are trying to pay the invoice? Below is my test in Cronus using your example with the Exchange Rate at Invoice of 2 and a Payment Exchange Rate of 1:
The Exchange Rate table below for EUR shows on 2/1 a Relational Exchange Rate = 2 which is for every 1 Euro = 2 LCY. My payment date is 3/1 and change this to Relational Exchange Rate = 1.

with this entry 
On Payment Journal we will pay the Invoice in LCY:
with posting equal too - 
I skipped the step to Adjust Exchange Rate at month-end but I normally would do this to close my books to calculate the unrealized FX Gain/(Loss). The above entry, for the Payment Journal, is a Realized FX Gain(Loss) since I am actually making a payment.
Hope this explains what the process is and steps.
Thanks,
Steve