ていては means "if you keep doing ~", and what follows is the bad result: something won't happen, or something will go wrong. It goes after the て-form of a verb, and it is a warning or a criticism.
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If you keep just complaining, nothing will change.
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If you keep being late every day, you'll get fired.
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If you keep eating that much, you won't be able to lose weight.
ていては or ては
on its own is a one-off "if you do ~" with a bad result. ていては says the action is going on, or keeps being repeated, so the warning is about a habit or a state that continues.
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If you quit now, it will all go to waste.
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If you keep taking days off, the work won't get done.
The second half is always something unwanted. A good result needs a different conditional.
- × 毎日練習していては、上手になる → 毎日練習すれば、上手になる
Casual てちゃ
In speech ていては shortens to てちゃ. It is not the same as the ちゃ that comes from ては: is "if you cry", is "if you keep crying".
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If you keep crying, you can't move forward.
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If you keep worrying about things like that, you won't get anything done.
Lingaku