When a program is told to expect input in a certain format, e.g. certain alphabet or numeric characters, and it receives something else, it will return the invalid argument error. The most common reason for this error in TaxAct is that it's receiving input for a language other than English.

Make sure Windows Region and Language settings are English. There are several settings to check: Location, Keyboards, Languages, and "Change system locale" under the Administrative tab. Consult help for Windows for these settings.

Microsoft Windows language settings also use the Ctrl + spacebar command to switch from one language setting to another, so you can inadvertently switch the language setting. There is a known Windows issue where even if you've removed another language, this key command can still switch off the language setting. You may have a language bar, possibly in the system tray, that will show if you've switched from English to another language.

You may need to consult information for any other software you have, including security software such as Kaspersky and "ease of access" software, that can affect language, input, or keyboard mapping. Refer to your operating system or software resources for help on these settings.

If you are using the online version of TaxAct, you may also need to check the language settings in your browser. Consult the help or support resources for your browser for how to do this.