import React from 'React'

should be

import React from 'react'

You are trying to import React instead of react. The name of module is case-sensitive, in this case the name is react.

Notification.js

import React from 'react'

const Notifications = () => {
return(
    <div>
        <p>Notifications</p>
    </div>
)
}

 export default Notifications

ProjectList.js

 import React from 'react'

 const ProjectList = () => {
 return(
    <div>
        <div className="project-list section">
            <div className="card z-depth-0 project-summary">
                <div className="card-content grey-text darken-3">
                    <span className="card-title">Project Title</span>
                    <p>Posted by Net Ninja</p>
                    <p className="grey-text">3rd September, 2018</p>
                </div>
            </div>
        </div>
    </div>
)
}

export default ProjectList

module name is react not React and since imports are case-sensitive import React from 'React' is causing error

I had the same problem and it was because I imported React not react so it should go like this ..

import React from 'react'