qt6_add_axserver_executable() and qt6_add_axserver_library() both contain the following call at the end:

qt6_target_idl(${target} ${arg_NO_AX_SERVER_REGISTRATION})

But arg_NO_AX_SERVER_REGISTRATION contains a boolean TRUE or FALSE value. The qt6_target_idl() function expects a keyword NO_AX_SERVER_REGISTRATION to be either present or absent, not an arbitrary TRUE or FALSE value to be provided as a positional argument.