error: “failed to delete old unity installation field. Maybe unity or some of its tools are still running”.

may actually be caused by insufficient permissions at installation time.

you can go to C:\Users\ \AppData\Local\Temp\unityhub- c187c6a0-6446-11e9-c3afd060978d to find the installation package downloaded by Unity Hub and install it with the administrator authority. The portion of the underlined path varies from person to person.