There are 2 things you need to check.
1. Global Settings in ShopFactory allow decimals
2. You Computer's Region Settings for Currency allows Decimals
In ShopFactory
a) Select Settings =>Global Settings from the Menu
b) After making Changes Click on Apply and then Click
c) Add another product and try to add decimals. Does this work now?
If Yes => Now make sure you turn on Rebuild All, it is under the Settings menu and must be ticked. Save & Re-publish your website
ShopFactory uses your Computers Region settings for digits and numbers
Check Windows Control panel (the location will vary depending on your version of Windows)
Go to Region and Language Options and make sure
i) the decimal and thousand separator are not identical
ii) the number of decimals is set to 2 or more
Please contact Microsoft Support if you need help with windows settings.
Finally If the windows settings need to be changed, please restart ShopFactory after you have made the changes.