This is the most common point of confusion. The v1.5 update is, in practice, the U-Boat Missions Add-on (a paid expansion).
You are likely using the Steam version of SH4.exe which is write-protected. Navigate to the folder, right-click SH4.exe > Properties > Security > give "Users" Full Control. Then re-enable mods. Silent Hunter 4 1.5 Patch Download
Do not waste hours on broken abandonware links or sketchy forums promising a 100MB file. The genuine 1.5 experience is worth the price of a coffee. Buy the Silent Hunter 4 Gold Edition on GOG, or grab the U-Boat Missions DLC on Steam. This is the most common point of confusion
