Callouts are not permitted from synchronous trigger execution.

Problem Explanation

Salesforce restricts HTTP callouts from certain contexts, including standard triggers and some Flow actions.

Root Cause(s)

1. Callout inside trigger

2. Callout inside non-async Apex

3. Mixed DML before callout

Step-by-Step Solution

1. Move callout logic to @future(callout=true) or Queueable

2. Invoke async method from trigger

3. Ensure no DML before callout in same transaction

Edge Cases & Variations

1. Platform Events allow callouts asynchronously

2. Named Credentials simplify auth handling

Common Mistakes to Avoid

1. Calling APIs directly from triggers

2. Ignoring transaction order