Yes you can bill for it since it was done in its entirety. It's not unusual to get unreliable results for a fields test due to various reasons.
If the field test is repeated again with the same unreliable results, you can bill for that as well, as long as you do the Interpretation and Report for it and include the test results in the exam record.
After two unreliable test, I usually recommend that the test NOT be repeated again.
Tom Cheezum, OD, CPC, COPC