Try `npm install @types/chart.js` if it exists or add a new declaration (.d.ts) file containing `declare module 'node_modules/chart.js';` code example

Example: could not find a declaration file for module

//First try  ( works for most of the cases) 
npm install @types/module-name

//if that does not solve your problem than . (will surely solve your problem)

Inside the project create a folder called @types and added it to 
tsconfig.json for find all required types from it . So it looks somewhat 
like this -

  "typeRoots": [
            "../node_modules/@types",
            "../@types"
        ]
        
And inside @types folder create a file called alltypes.d.ts .To find the unknown 
types from it . so for me these were the unknown types and I added it over there.

declare module 'react';
declate module 'react-dom' ;

So now the typescript didn't complain about the types not found anymore .