score:1

const italic = styled.text`
  font-size: "12px";
  lineheight: "20px";
  text-align: left;
  font-family: "roboto condensed";
  letter-spacing: "0.5px";
  font-style: "italic";//problem is with this line
  font-weight:400;
`;

where ever you are suffixing px needs to be in either single or double quotes and font-style: value(italic) need to be in double quotes as well.

score:6

when you register your fonts, you need to make sure to include a variant for each fontstyle you wish to use. for example:

font.register({
  family: 'roboto',
  fonts: [
    { src: '<path-to-normal-font-variant>' },
    { src: '<path-to-italic-font-variant>', fontstyle: 'italic' },
    ...
  ]
});

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