When resubmitting an online claim through SimplePractice, we try to automatically include the resubmission reference code. Sometimes we're not able to auto-populate this code because the previous claim has been deleted.
In this case, you can try submitting the claim as a new claim so the resubmission code is not required. Many payers accept this. You can indicate the claim is a new claim by adjusting box 22 on the CMS form.
If you do need to find the reference code for a previously filed claim, you can manually look it up by going to the Insights section and clicking on the Electronic Insurance Claims report.
Here you can find the initial claim and find the reference code for resubmission: