1. Skills vs Target Scores
Your current level is way lower than what you need; you have to be realistic on the amount of time you need to prepare for each section. It is important to be confident but realistic too if you can really achieve your scores in just short period of time. Otherwise, it would mean wasting money, time and effort for the test.
2. Clueless Preparation
This is the most common mistake that people have. They do not know what to prepare and how to improve. Perhaps, this is the most critical because if you don't know what you're doing, you'll end up with the same scores over and over again. Do not answer random tasks; focus on what are you trying to achieve per task. It does not matter how many tasks you answer. You could end up finishing all TPO but getting the same results.
3. Inconsistent
Some people almost made it at some point, 1 or 2 points away. However, we don't get to apply these skills often in our daily life or jobs. For instance, logical reasoning, which is important in answering reading questions, would be hard to apply in real-life situations especially if your job requires more technical skills. Your ability to explain complicated or complex concepts, critical in speaking task 3 and 4, is not something that we always do in our daily convo.
4. Most websites DO NOT provide accurate scores you could get in the actual test.
Just because you got a high score by answering some tasks from whatever website does not mean it'd be the same. You need to take into account the difficulty of the actual test (some are good though just for practices but do not depend too much on the scores; you might be disappointed). Going back to what I've been trying to point out, you need to prioritize the quality of your training or practices.
5. Too much info or unnecessary info
In the past 5 years, TOEFL has changed its test format and even the parameters they prioritize when grading, for instance, your speaking and writing sections. There's too much info on Youtube giving you tips that might not be applicable today. Just because one of your friends did this or did that does not mean you'd get the same high scores. Too much unnecessary info is harmful.