There are two ways to do this. The first is if you don't want the attachment synced to the invoice. An easy fix would be to disable this feature. To do that navigate to Setup QBO in Mobius >> Sales tab>> Disable transfer attachments>> Then you can run the batch again.
Now if you do wish to use sync the attachment to QuickBooks Online and this error is stopping that from happening the most likely cause of this is a space in the name of the API Member used to connect Mobius to ConnectWise PSA. For example, if the API Member's name is "Mobius Test," there must be no space between Mobius and Test else the error will occur. Instead, the name should be "MobiusTest" without any white spaces.