It would be easiest for us to troubleshoot this if you provided an example of your request in full (making sure to keep your keys private).
Here is an example of a curl request that you could use to hit this endpoint:
curl --request POST \ --url https://api.twitter.com/1.1/tweets/search/fullarchive/{ENVIRONMENT_NAME}.json \ --header 'authorization: Bearer {BEARER_TOKEN}' \ --header 'content-type: application/json' \ --data '{ "query":"from:TwitterDev lang:en", "maxResults": "500", "fromDate":"{YYYYMMDDHHmm}", "toDate":"{YYYYMMDDHHmm}" }'
Just replace {ENVIRONMENT_NAME}, {BEARER_TOKEN}, and {YYYYMMDDHHmm} with your relevant information.
If you are still experiencing issues with the bearer token, please use the following cUrl command:
curl -u '{API_KEY}:{API_SECRET_KEY}' \ --data 'grant_type=client_credentials' \ 'https://api.twitter.com/oauth2/token'
Making sure to replace {API_KEY} and {API_SECRET_KEY} with those tokens that are associated to the Twitter app that you have associated with this product in the dev environments page.
Might I also suggest that you try playing around with Insomnia? It has worked very well for my team in making requests to REST endpoints.