Web Server OAuth Authentication Flow

The server responds with a 401 with the following body:

[{"message":"Session expired or invalid","errorCode":"INVALID_SESSION_ID"}]

Probably the wrong instance URL, change from https://xxyy.lightning.force.com/services/data/... to https://xxyy.salesforce.com/services/data/... e.g change from lightning to salesforce.

The second solution use relaxed IP restriction.

https://success.salesforce.com/answers?id=9063A000000ZtvGQAS

Apex Triggers is not visible

You can not find the Apex Trigger section

You need create a new account in salesforce to developers, the account which you uses is a production account, the production accounts don´t have permissions to create a new Apex class and Triggers, i let the link to signup developer account in salesforce bellow. here I hope this comment help you.