Hi mate
With regards to that message you got around the payment ID, if you go to any exchange that accepts Monero you can just go to deposit Monero then it will generate a payment ID for you. This payment ID stays valid forever.
MAKE SURE YOU DOUBLE CHECK THE ADDRESS AND PAYMENT ID !
You can then just send your monero over to the exchange and sell it for BTC. If you are using a command line wallet:
1. run the CMD and get the blockchain downloaded
2. Open you wallet and wait for it to sync
3. type in "save" on both the cmd and wallet so you dont have to wait long next time
4. type in "transfer" <public address> <Amount> <payment ID>
The syntax i gave on .4 might be the wrong way around but it wont let you transfer to a payment ID so you can type in "help" to confirm the syntax, if it stops you then just switch it around - you can check vids for electroneum wallet CLI transfers too, its a monero fork so the same will work (ofc make sure to use monero wallets and stuff then dont download electroneum wallets and send your monero there)
5. "type in show_transfers" in the wallet CLI to see the transfer and if it has cleared
6. wait about 5-20 minutes and it should be cleared
Hope the above helps you. If you have your private keys then you can just download the wallet CLI. Make sure you are getting the correct one - I wont be pasting the link here as I myself dont trust any links people share on bitcointalk
cheers