Testing auth not covered case in sandbox
According to plaid docs, plaid does not support auth for 10% of bank institutions.
Is there a way to test this on sandbox environment?
Yes, there is! There is a document with all the details on the Plaid site Testing all Auth flows.
The full support version of Auth, that works with 90% of banks, is called Instant Auth. The other flows are called Instant Match, Automatic Micro-deposit Verification, and Same-Day Micro-deposit Verification.
To test Instant Match and Automatic Micro-deposit Verification, you use the test institution Houndstooth Bank and then pick specific test accounts as described in the article. I recommend just reading the article because it has all the details, but here's the summary version:
For Instant Match:
Search for “Houndstooth Bank” in Link. Enter user_good and pass_good in the Credential pane. Select the second account that is returned: Plaid Savings (****1111). In the Routing number input, enter: 021000021 or 011401533 In the Account number input, enter: 1111222233331111
For Automatic micro-deposits:
Search for “Houndstooth Bank” in Link. Enter user_good and microdeposits_good in the Credential pane. Select the first account that is returned: Plaid Checking (****0000) In the Routing number input, enter: 021000021 or 011401533 In the Account number input, enter: 1111222233330000 Enter your legal first and last name. Select personal as the account type. Link will display the Automated Micro-deposit success view – click continue to trigger the onSuccess callback with a public_token. The micro-deposit verification will automatically succeed after twenty-four hours. To test a failed micro-deposit, or to skip the twenty-four hour waiting period, use the /sandbox/item/set_verification_status endpoint to manually control the Item's micro-deposit verification status.
For Same-day Micro-deposit Verification (this is the flow for banks that really can't connect to Plaid):
Search for an institution, and scroll to the bottom of the search results, click on connect your bank manually. Select “checking” or “savings” as the account type. In the Routing number input, enter: 110000000 In the Account number input, enter: 1111222233330000 Link will display the success view – click continue to trigger the onSuccess callback with a public_token. To verify the deposit, call /item/public_token/exchange with your public_token from the previous step to receive an access_token. Call /link/token/create and provide the access_token from the previous step to receive a link_token. Open Link with your link_token. In the first deposit input, enter $0.01 In the second deposit input, enter $0.02