Can't really tell the line number because you cut out half of your code. Please paste the whole BuildSystem.cs if the error persists. It's saying your BuildSystem script refers to something called 'Block' on line 24, but 'Block' doesn't contain any field or property called 'IsSolid'.
https://support.unity.com/hc/en-us/articles/206193836-What-is-CS1061- CS1061 occurs when you try to call a method or access a class member that does not exist.
Yeah, as @Mauri says. Your Block class doesn't have anything called IsSolid that your BlockSystem script can access on line 122.
What I saw in that script: The currentBlock.isSolid at line 83 has a small i, whereas the one at line 122 (where the error happened) has a capitalized one.