Lambdacism is actually the opposite: it refers to the substitution of the "L" sound for other sounds, like "w" or "r".
The term for being unable to pronounce the "L" sound is L-dropping or Lateral lisp. This is a speech disorder where someone has difficulty producing the "L" sound correctly, often substituting it with other sounds.
It's important to remember that language and speech disorders are complex, and these terms are used specifically in these contexts.