
When logging into the Microsoft Planner mobile app with an Office 365 work account it displayed the error “You can’t get there from here”.
Checking Conditional Access policies and Intune App Protection policies confirmed that access should be allowed.
The issue was due to hidden classic Conditional Access policies being applied in conjunction with the new Conditional Access policies. The classic policies can be found at the URL below:
https://portal.azure.com/#blade/Microsoft_AAD_IAM/ConditionalAccessBlade/ClassicPolicies

Disabling the classic policies then allowed a login from the mobile app.