When it does not work

abpv doctor
abpv doctor --token "$ACCESS_TOKEN"   # also compares the permission names
abpv doctor --offline                 # only what can be told without the backend

Nearly every "the frontend cannot talk to the backend" is one of a handful of mismatches, and all of them are mechanically checkable. Every failure prints the command that fixes it.

Check	What it means when it fails
Environment	Node, the package manager, the .NET SDK, the ABP CLI
Backend reachable	GET {api}/api/abp/application-configuration
Certificate	The development certificate is not trusted: dotnet dev-certs https --trust
CORS	Your origin is not in CorsOrigins on the host that answers
OpenIddict client	The identity server's discovery document, and whether it knows your clientId
Redirect URI	Your configured redirectUri is not among the client's RedirectAllowedUrls
Versions	Which ABP the solution was generated for, against what this release is tested with
Proxy freshness	The generation is run again in memory and compared file by file
Object extension coverage	The backend declares N extension properties and the mapping rules recognise M
Released source	Which packages no longer follow releases

The object extension line

Fifteen rules turn a property the backend declares into a column and a form field. Missing any one of them looks exactly like a mistake in your own configuration — the property is configured, and nothing shows. So doctor puts both numbers next to each other and names the difference:

⚠ object extensions   backend 9, recognised 8
                      not recognised: IdentityUser.HireDate (DateTime?, hidden on the table)
                      → this is a gap in our mapping rules; please open an issue

If it names a property, it is our bug, not yours.

The usual first-run ones

A blank page and a network error	The backend is not running, or the certificate is not trusted
Backend requests return 500 with "The Libs Folder is Missing"	Run abp install-libs from the HttpApi.Host project directory, then restart its host
Sign-in loops back to the login page	The redirect URI does not match what the OpenIddict client was seeded with. abpv switch-ui --port <yours>, then run the DbMigrator again
Every request is 401 after switching tenants	Expected once: the token was minted for the other tenant and is discarded. If it keeps happening, the tenant's own client is not seeded
The menu is empty after signing in	The user has no permissions, or application-configuration is being served from cache. Hard reload
A column the backend configured does not show	abpv doctor and read the object extension line

Asking for help

Open an issue with the output of abpv doctor in it. It says what the toolchain is, what the project thinks its backend is, and which check failed — which is most of what anyone would ask you for anyway.