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Query Unterminated errors are due to the instrument not seeing the expected termination character in the SCPI string. The instrument is probably smart enough to process the commands ending in '?' without a termination character since in SCPI, there can't be anything after the '?' other than a termination character.
[Aside: I find that Keysight instruments- formerly Agilent, formerly Hewlett-Packard - handle missing terminations very well. Other manufacturers do not - there can be timeouts, or for some instruments using SCPI over USB, ther can be a complete bus hang requiring a power/cycle or unplug-replug to get communications going again.]
Most instruments expect either a newline or carriage return as a termination character. Try adding a \n or a \r to the end of your command strings in NI-MAX and see if the non-'?' commands work. You can also have NI-MAX automagically add termination characters.
To get an authoritative answer as to what termination character the instrument is expecting, review the E8631A programming reference guide. (I find it sometimes faster to just experiment with NI-MAX though.)
Also: when you are sending a non-query string, make sure you are just pressing 'send' because the instrument is not going to respond with anything. Sending it as a query (write/read) will cause a timeout. Normally if I am just sending a non-query command, I will either append ';SYST:ERR?' to the end or send 'SYST:ERR?' as a separate query, to know that the standalone command actually worked.
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