Do not remove the template without fixing the problem one of the following ways.
If you know the place, fill in the needed information, and remove the template.
For a template-formatted citation, add the place
|place=XXX
For a free-form citation
:
Just add the place as appropriate to the format of the citation; or...
Better yet, convert the entire citation to {{cite journal}}, {{cite news}} or some other {{cite xxx}}-series template, as appropriate for the work in question.
If you know that no place was specified by the original source, as is common with older books, you have the following option.
Explicitly state that the location was unspecified:
|Place=<!-- Place unspecified --> (you must use HTML<!-- -->comment tags to hide the message from the citation processing software)
For free-form citations:
No place specified.
Do not use question marks.
Do not leave the information blank and untag it, or someone else will just come along later and flag this with {{Place missing}} again!
Do not use |Place=none, |Place=unknown or anything else vague; any implication other than that the source itself did not specify an ISBN is simply a signal to other editors to re-tag it with {{Place missing}}.
If you don't know:
Check the source, and add the necessary information, as above.
Do not use question marks.
If the source is a dead link, check archive.org for a backup copy (see your {{citation}}/{{cite xxx}}-type template's documentation for use of |archiveurl= and |archivedate= parameters). If no archive copy is available, use {{Dead link}} after the citation, but leave {{Place missing}} as well.