Is it true that if i don't blow my nose before a covid test, the result could make a false positive? is that the only error?

The idea is to sample cells lining the nose, not nasal mucus, so when COVID tests were first developed and being researched, tested people were advised to blow their noses first and it became a standard recommendation. But there has been little or no research on test performance with and without nose blowing. It's best to follow the instructions, but probably it makes no difference.

No. The test detects actual viral particles and won't be triggered by extra mucous. Most of the errors in the quick COVID test are false negatives when tested too soon. If you have a positive test, you can assume you have the virus, keep a low profile and stay home unless you become short of breath or develop persistent fever. If this occurs or you get sicker in any way seek medical attention.