trim audio and delete grayed out

hello there,
today I installed audacity and I tried to delete part of an audio file.
I selected the part of the file i want to delete and I see that the segment is marked with the gray arrow.when the part is selected, the play back starts automatically .at the very moment that the playback ends, the segment is not marked anymore.
the menu edit → delete is then greyed out.
if i activate the setting “select all audio in project if none is selected” then the menu is not greyed out but the entire file is deleted.
How can I keep the segment selected and successfully deleted ?
Thank you for your help !

info :

linux mint 18.02
audacity 2.1.2
installed from the package manager

To select the audio, click and drag on the audio itself (the blue wiggly waveform), rather than the Timeline above the tracks.
Note that if you have multiple tracks in the project, you can click and drag across multiple tracks to make the same selection in each track.
More information about selecting audio here:

Clicking, or click and drag, in the Timeline activates the “Quick Play” feature (see:

Thanks a lot ! it worked like a charm