In the translation window you have buttons for plural option for the string. You need to give translations for both. Just click "other" and fill in the plural form of the string (otherwise use same as for singular).
Date: Fri, 1 Sep 2017 15:09:11 +0000 (UTC)
From: Dick Groskamp <[hidden email]>
To: QGIS Translation Discussions <[hidden email]>
Subject: Re: [Qgis-tr] Impossible to translate a string
Message-ID: <[hidden email]>
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it has probably to do with the number of translations.Yhere should probably be two:
one for a singel object
one voor multiple objects
The reason why it is noy savinf is that youy have not filled one of the two possibilities""Fill in all plurals to continue"
In the upper right corner of the translation pane of Transifexit syas  [other]
Clicking  or [other] switches between the two options
If both options are filled you can save the translation
Van: Stefano Campus <[hidden email]>
Aan: [hidden email]
Verzonden: vrijdag 1 september 15:04 2017
Onderwerp: [Qgis-tr] Impossible to translate a string
Hi, I'm trying to translate this string of QGIS-application in italian, but Transifex tells that there is an error in the original english phrase
ERROR: %n feature(s) not added - geometry type is not compatible with the current layer.
it's not possible to save a translation "Fill in all plurals to continue".
I'm translating from English to Italian. Is there any other language which has same problem?
stefanoTransifex Webtranslation page for QGIS is on https://www.transifex.com/qgis/