On another forum post, similar to this one, here is a solution that works: Step 1: Go to the sandbox page. Step 2: Refresh the page. If this step is not completed, proceed to the next step. Step 3: Turn off Wi-Fi or any network connection, then click on the sandbox. You will see an error on the sandbox page along with an "Add Test Account" option. Click the "+" button to open the Add Account modal. Next, turn on Wi-Fi or the network connection while submitting. You are good to go!

Same problem and the browser gives this error: TypeError: Oe.totalAccounts is undefined

Same here, and here is the error shown in the browser console: TypeError: Oe.totalAccounts is undefined Yp https://appstoreconnect.apple.com/access/f5a40a23f0a74c578deb79889c64591c/main.f25e31519101c5aa2654.bundle.js:2 aa https://appstoreconnect.apple.com/access/f5a40a23f0a74c578deb79889c64591c/main.f25e31519101c5aa2654.bundle.js:2 Ha https://appstoreconnect.apple.com/access/f5a40a23f0a74c578deb79889c64591c/main.f25e31519101c5aa2654.bundle.js:2 qs https://appstoreconnect.apple.com/access/f5a40a23f0a74c578deb79889c64591c/main.f25e31519101c5aa2654.bundle.js:2 Pl https://appstoreconnect.apple.com/access/f5a40a23f0a74c578deb79889c64591c/main.f25e31519101c5aa2654.bundle.js:2 Ol https://appstoreconnect.apple.com/access/f5a40a23f0a74c578deb79889c64591c/main.f25e31519101c5aa2654.bundle.js:2 kl https://appstoreconnect.apple.com/access/f5a40a23f0a74c578deb79889c64591c/main.f25e31519101c5aa2654.bundle.js:2 yl https://appstoreconnect.apple.com/access/f5a40a23f0a74c578deb79889c64591c/main.f25e31519101c5aa2654.bundle.js:2 Ko https://appstoreconnect.apple.com/access/f5a40a23f0a74c578deb79889c64591c/main.f25e31519101c5aa2654.bundle.js:2 unstable_runWithPriority https://appstoreconnect.apple.com/access/f5a40a23f0a74c578deb79889c64591c/main.f25e31519101c5aa2654.bundle.js:2 qo https://appstoreconnect.apple.com/access/f5a40a23f0a74c578deb79889c64591c/main.f25e31519101c5aa2654.bundle.js:2 Ko https://appstoreconnect.apple.com/access/f5a40a23f0a74c578deb79889c64591c/main.f25e31519101c5aa2654.bundle.js:2 Wo https://appstoreconnect.apple.com/access/f5a40a23f0a74c578deb79889c64591c/main.f25e31519101c5aa2654.bundle.js:2 pl https://appstoreconnect.apple.com/access/f5a40a23f0a74c578deb79889c64591c/main.f25e31519101c5aa2654.bundle.js:2 _a https://appstoreconnect.apple.com/access/f5a40a23f0a74c578deb79889c64591c/main.f25e31519101c5aa2654.bundle.js:2 qd https://appstoreconnect.apple.com/access/f5a40a23f0a74c578deb79889c64591c/main.f25e31519101c5aa2654.bundle.js:2 928.d9809529311b483838fc.js:2:13441