welcome to the wonderful world of GRASS.
The first thing I can think of that you have installed GRASS separately as v. 7.4.1 and installing the v.centerline there?
Usually, QGIS 2.18 comes with GRASS 7.2 as built in with it, at least that is how it is installed in my system. You can check it by opening the GRASS shell (Marked as 1 in the first screenshot) inside QGIS and typing g.version in the console. If that is the case, you have to open the QGIS associated version from QGIS folder in your Start menu and install the extension there. Anything available from GRASS in QGIS should appear if you search in the Filter box (marked as 2) or type the module name in the GRASS shell I mentioned.
Another point to check is where your extension is getting installed and point the GRASS-QGIS to that directory from the following GRASS option window. I got that by right-clicking on the mapset I opened and selecting 'GRASS Options'
Also, note that the extensions in independent GRASS installation in Windows create an .exe file with the name of the extension and GRASS-QGIS uses GRASS modules as .qgm format from 'C:\your_path\QGIS218\apps\qgis\grass\modules'.
In my opinion, you are really better-off using GRASS independently and bring the output of your analysis into QGIS for further processing.