Serial communication slight problem using Wemos D1 R2 and UNO + ThingSpeak

AndiAkF:
i think its fine not to empty it since it reset the index to 0,

Yes

and how do i identify which data i want it "receivedString" array?, the code above is just a guess.

If you print the complete contents of receivedString[] what do you see - post some examples.

...R