Tried alot of fixes and none seem to work, seems like alot of people on these forums have the same problem, but no fix yet, also when typed in google loads of people have this problem and no fix.
My PC is XP 2700+ cpu based.
Do you have Source SDK Base 2007 installed ?.
And do you have the mod installed in Steam\steamapps\sourcemods\BMS ?.
This doesn’t really sound like a game bug, more a user issue.
Yes to all of the above, can run the SDK demo as well, can run all other steam based source games ok.
I have the same problem with me, it starts then it crashes and there is the error message cound not found load libery client
I have done what has proposed steam unzwar the game info in steamad 215 but it does not remain so goes the message got ego regulations gemäs like the way it is in the description changed again to 218 and it still does not work I desperate 
my pc is technically good enough source for css running on the highest graphic utilization without falter I need help
mfg aimmonstaa
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Known problem. Athlon XP CPU do not support SSE2 instructions set which is required by BM. No luck for people having these CPUs unless devs would agree to release a separate version of client.dll/server.dll compiled with SSE2 usage disabled.
has that been confirmed any where ? thx
Here is the thread dedicated for this problem (hint use search button first prior starting a new thread hint):
https://forums.blackmesasource.com/showthread.php?t=12671
This problem hadn’t been officially confirmed by devs but you could be 99% sure it’s the cause. Look up investigation details in the thread I posted a link to above.