The payer ID that they are referring to is the EDI payer ID. This is the ID that is used by the clearinghouses and the insurance companies to route the electronic claims to the correct insurance company. Kind of like their electronic address. This error is saying that the payer ID that Therabill has on file for this payer is no longer valid. They may have changed their payer ID or they may have discontinued electronic claims.
How to fix
E-mail firstname.lastname@example.org and let us know that you received an error stating "Invalid Payer ID" and let us know the insurance company name. We will investigate and hopefully update our records with a new payer ID.
If it turns out that we can no longer submit electronically to this payer, then you will need to change your claim to a paper claim and submit to them through the postal service.