Hi Yusuf,

The codec problem usually comes when you have a corrupted video file or may be the settings configured in your PC is wrong.
Here are some of the reasons for this problem.

Reason 1: The MP4 video is fake or corrupt
Reason 2: Maybe the VLC "video output modes" is set wrong.
Reason 3: If an MP4 file contains a VLC incompatible video codec, it will fail to play in VLC.

You can try the below fixes in order to fix his issue.

Fix 1: In this situation, you need to configure X Server correctly:
Open VLC Player, then go to Tools > Preferences.
In the Video tab:

• Check the box for Enable video (if it's unchecked).
• Check the box for Window decorations (if it's unchecked).
• Set the Output drop-down menu to X11 video output (XCB).

Finally click "Save" to save your changes and restart VLC player.

Fix 2: Find a more compatible MP4 player or convert MP4 to VLC more supported video format.
iDealshare VideoGo is just the ideal solution which can act as an ideal MP4 Player or MP4 VLC Converter.
Besides playing or converting VLC compatible video and audio format, it also can play or convert VLC unsupported formats like WTV, DVR, VOB, WebM, OGV, F4V, MVI, MXF, AVCHD, MTS, M2T, M2TS, MOD, TOD, Apple ProRes, R3D, M4V, ASF, TS, AIFF, M4A, Apple Lossless, 3GA, QCP, CAF, MPC, VOC, SHN, TTA, AU, RA, M4B and so on.

Step 1 Import MP4 video
Click the "Add File" button to add MP4 files which are not playable in VLC player to this VLC Converter app.

Step 2 Set VLC supported format
Click the "Profile" drop-down button to select VLC supported video format from "General Video" Category or "HD Video" Category.
To convert MP4 to VLC supported audio format, select VLC compatible audio from "General Audio" Category or "Lossless Audio" category.

Step 3
Click the "Convert" button to start and finish converting MP4 to VLC more supported format to successfully play MP4 in VLC.

Fix 3: Update VLC
Although VLC is supposed to update automatically, but try this.
Open VLC > click Help > click " Check for Updates ".

I hope this helps.

Regards,
Lokesh