Hi!
I can’t do a full scans because the button is unconfigured. In its place, appears a small flag and the text “scans.scanType.undefined”. Of the 4 buttons for scan 2 are with this problem:
Does this help? >> https://support.avast.com/en-us/article/Repair-Antivirus
I really appreciate your help, but this alternative did not solve the problem.
Are you using the latest version ???
https://forum.avast.com/index.php?topic=215365.0
If you are and repair didn’t work, try the following:
Clean Install of Avast:
https://goo.gl/4Ptzkf
It may be relevant to say that I am having many disk errors at boot of Windows10pro (v 1709, comp 16299.192). I can only log into the system after leaving the computer off for about 40 minutes. Once entering Windows, everything runs fine, and Chkdsk does not detect any disk problem, except that I can’t do Avast scans without the button for this. I do not think it’s a temperature problem since no overheating is indicated by the BIOS or softwares like Aida, Speecy, HDDThermometer.
My Avast version is 18.1.2326, build 18.1.3800.0, virus definition 4,192,553. If I do a clean install of Avast, I’ll probably stay a long time without posting here because of startup errors and delays.
Maybe you need to sort out the underlying problems first. What other security related programs are installed.
Thoroughly check your HD for possible problems: https://www.lifewire.com/how-to-scan-a-hard-drive-using-error-checking-2624497
Only WindowsDefender and Avast. Glary Utilities could be in this category because its last update installed MalwareHunter without my waiting, which I uninstalled four days ago on suspicion. I’m still with Glary.
Have no idea what changes Glary may have made. I don’t use it. Ccleaner does my cleaning of garbage but,
I don’t allow any program to make changes to the registry in order to do a “clean-up”.
Check your HD.
I only use Glary for monitoring, I like to do the cleanups and maintenances manually so do not have problems with lag of versions between programs. Thank you for link with list of 14 Free Hard Drive Testing Programs. Some I already knew, but the most interesting ones have not. I’m going to do my tests, reinstall Avast clean and re-post here tomorrow or later if I still have a computer ;D
Many thanks for all informations.
Thank you all, I really appreciate your help. The clean reinstallation of Avast has returned the buttons correctly and I can re-enable scans as before. I may have inadvertently aborted an Avast update before it was completed. The problem of disk errors already existed before the first installation of Avast and continue after solved the problems of the buttons, therefore without any relation with the antivirus. I’ll see what I can do with the programs indicated for HDD repair.
You’re welcome. It sounds like it’s time to invest in a new HD. 
This HD is already an investment of 40 days ago, I think I need to invest in a lawyer >:(
??? ??? ???
This re installation of Avast will not permanently fix the problem and it is unlikely it has anything to do with disk errors. It will re occur again at some point… It is a bug…that Avast need to fix. Do a backup of your settings at bottom of General Settings page when problem does not exist and you have all configurations how you want them. When the problem happens again which it will inevitably then restore the settings. The problem will show when it happens as it did with you but it may also remove any Customs scans you have created. The problem is Avast seems unable to retain settings for some components. Gary