My plan is actually creating a transaction system on a third-party software.
Im using Bitcoin Core to be able to avoid any ratelimits, downtime, etc.

And your answer and the answers above I fixed it!

Thanks!!!

Oh got it! But im working on a program which will be creating a lot of new wallets etcetera. How would i be able to load all of these when for example starting Bitcoin Core?
And how would I about loading the wallet when i create it.
All of this should be through RPC ^^.

Yep thats the library im using! And if you're wondering if it's the library that's making it not work, i tried with just making normal POST requests to the RPC. Still getting the same error:

{"result":null,"error":{"code":-18,"message":"Requested wallet does not exist or is not loaded"},"id":null}

And this is my test code with requests

----
import requests, json

headers = {'content-type': 'application/json'}
payload = json.dumps({"method": "getwalletinfo", "params": [], "jsonrpc": "2.0"})
response = requests.post("http://myusername:mypassword@127.0.0.1:18332/wallet/testingwallet", headers=headers, data=payload)

print(response.text)
----

I have been playing around a bit with Bitcoin Core (version 23.0), more specifically connecting to different wallets in Bitcoin Core through RPC in Python. I have found this library and tried to connect to my wallet(s) using this code:
--------
from bitcoinrpc.authproxy import AuthServiceProxy

conn = AuthServiceProxy("http://myusername:mypassword@127.0.0.1:18332/wallet/testingwallet")
print(conn.getwalletinfo())
--------
But when running this i get this stacktrace:
-----------
Traceback (most recent call last):
  File "/home/kebab/Desktop/pron/testing.py", line 24, in <module>
    print(a.getwalletinfo())
  File "/home/kebab/.local/lib/python3.8/site-packages/bitcoinrpc/authproxy.py", line 141, in __call__
    raise JSONRPCException(response['error'])
bitcoinrpc.authproxy.JSONRPCException: -18: Requested wallet does not exist or is not loaded
-------
Though what I have noticed is that if i load the wallet using bitcoin-cli -testnet loadwallet testingwallet. Then i can run RPC commands but only get info from that wallet!

And I have created my wallet using: bitcoin-cli -testnet createwallet testingwallet

Any help on this would really be appreciated!