Meta WhatsApp Cloud API – Technical Helper Required (Short-Term, Task-Based)

I am trying to integrate WhatsApp Cloud API–based system to my ERP. My ERP is currently failing on OAuth failure with message :

✗ API Connection Failed (HTTP 401)
Response: {
"error": {
"message": "Invalid OAuth access token - Cannot parse access token",
"type": "OAuthException",
"code": 190,
"fbtrace_id": "AyFYHTpf4C5HjqIsXGKXALd"
}
}

❌ 401 Unauthorized - Token is invalid or expired

I already have tried getting new meta API tokens and updating details but have been unable to access yet.

I already have my business verified, Phone Number verified, Message templates already approved as Utility on Meta. Only App Review is under verification.

I am looking for a technical helper.

Scope of help :
Debugging Meta Graph API / WhatsApp Cloud API issues
Token types (System User vs User vs App), permissions, Business Manager mapping
Message send / receive flows, templates, webhooks
Reviewing existing PHP (PDO-based) implementation
Explaining why something fails, not just “try this”

Important clarifications
Backend is already developed; this is assistance + troubleshooting, not fresh development
No UI / frontend work
No reselling, no client communication, no long-term dependency
I prefer short, focused sessions or task-based help

Tech stack
PHP (PDO)
WhatsApp Cloud API
Webhooks
MySQL

What I expect from you
Prior hands-on experience with WhatsApp Cloud API (not just reading docs)
Clear explanations and structured thinking
Ability to work independently once context is given
Honesty if something is not possible or restricted by Meta

Engagement model
Contractual
If you have real experience with Meta WhatsApp Cloud API, send:
Brief summary of similar work done
What part of WhatsApp API you’ve handled (tokens, templates, onboarding, etc.)
Please do not apply if you are learning this for the first time.