You likely have a cracked repack or an offline backup. If you own the game legitimately on Steam, SCS Software does not allow "downgrading" to 1.34 via official betas (they only support back to 1.43). Therefore, v.1.34.0.4 18 DLCs exists primarily in archival communities.
Warning: Mods for 1.34 are hard to find. Most modders have updated their trucks to 1.50+. However, the included 18 DLCs mean you don't need mods. The base experience is fully fleshed out. American Truck Simulator v.1.34.0.4 18 DLCs. F...
Note: The exact list can vary by repack, but the most common 18 DLCs for this version are: You likely have a cracked repack or an offline backup
Missing from this version (compared to modern releases): Utah, Idaho, Colorado, Wyoming, Montana, Texas, Oklahoma, Kansas, Nebraska, etc. Missing from this version (compared to modern releases):
Some veteran players argue that the economy and fatigue simulation in 1.34 is less forgiving and more realistic than later updates, which introduced overly lucrative quick jobs.
Some repack sites (e.g., GOG offline installers, certain archives) offer a pre-assembled v.1.34.0.4 with all 18 DLCs as a single package. Be cautious: only download from trusted sources to avoid malware. Legally, owning the DLCs on Steam is the cleanest method.