Accessing Intune Admin Portal with GDAP

I have the same error and was able to resolve it.

{
"sessionId": "40c71ee0c1544b10ab6058affe2d538c",
"missingClaims": "{\"claims\":\"{\\\"access_token\\\":{\\\"capolids\\\":{\\\"essential\\\":true,\\\"values\\\":[\\\"9deb8598-462b-462a-ac1c-478984b576dd\\\"]}}}\"}",
"resourceName": "manageddesktop",
"errorMessage": "AADSTS90072: User account '{EmailHidden}' from identity provider 'https://sts.windows.net/<tenantid>/' does not exist in tenant '<customer>.' and cannot access the application '5926fc8e-304e-4f59-8bed-58ca97cc39a4'(Microsoft Intune portal extension) in that tenant. The account needs to be added as an external user in the tenant first. Sign out and sign in again with a different Azure Active Directory user account\r\nTrace ID: <objectid>\r\nCorrelation ID: 6aabee5c-79d5-4341-a0ba-0934dc518bb3\r\nTimestamp: 2023-07-06 09:26:22Z"
}

After adding an indirect reseller relationship without DAP I was able to access intujne through GDAP. If you already have a reseller relationship in place, try deleting and readding the indirect partner relationship. Hope this helps.