In order for Twilio Rebilling to work, your customer's sub-account in HighLevel needs to be connected to a corresponding sub-account in your Twilio account.
If the client you want to activate rebilling for was using the same SID/Token as another client, you'll need to follow the following steps before activating Twilio Rebilling in SaaS mode:
Step 1 - Log into your Twilio account > click the Gear Icon > select sub-accounts > click the red + icon to create a new sub-account and name it to match the client you want to activate Twilio Rebilling for
Step 2 - Log into your HighLevel account > go to Agency Settings > Twilio > scroll down to your client > click the three dots icon to the right of their account name > select "Update Credentials" and replace the SID and Auth Token values with the new SID and Token values from the new sub-account in your Twilio account.
You can obtain the new SID and Token by logging into your Twilio account > click the gear icon at the top right > select Sub-accounts > Click the name of your client and you will see those two fields: