Hello Ade,
On the appliance you don't use full file path names.
Here is an example from my lab:
---
# The following files are currently available for this configuration entry:
# - spnego_V8.keytab
spnego-krb-keytab-file = spnego_V8.keytab
---
Once the keytab is recognized by the Reverse Proxy instance then you'll see it available as a comment in the configuration file. You just set the value as-is without a path. The paths are hidden on the appliance to prevent misconfiguration.