"达不到预想效果" can be translated into English as "fail to achieve the expected results" or "fall short of the intended effect".
Both expressions convey the idea that the actual outcome does not meet the anticipated or desired standard. Here are examples in context:
"Despite our best efforts, the project still failed to achieve the expected results."
（尽管我们尽了最大努力，项目仍然达不到预想效果。）
"The new marketing strategy fell short of the intended effect, and sales did not improve as hoped."
（新的营销策略达不到预想效果，销售额并未如预期般增长。）