You likely have a keyboard installed on your phone that does not support the comma. This can happen due to operating system updates.
This problem can be fixed by installing the default Gboard from the Google Play Store.
Click on the following link:
https://play.google.com/store/apps/details?id=com.google.android.inputmethod.latin&hl=en
After installation, you should be able to enter the weight with a decimal point.