a virus is blocking me to start avast scaning

https://support.avast.com/article/Remove-Antivirus-password

I suspect I know what your problem is. I thought I had previously reached out to Milos for comment. Is there a PUP attached as a prefix to the detection? (Eg: [PUP] Win32/Avast.Cleanup)

can you tell me what is a FP ?

FP = False Positive = a clean file detected as malicious

FN = False Negative = a malicious file detected as clean

https://www.howtogeek.com/180162/how-to-tell-if-a-virus-is-actually-a-false-positive/