Phil's answer is correct. And here is how to create .mvn directory with needed jar inside.
I found the answer here (https://www.baeldung.com/maven-wrapper)

If you have maven and you want to make maven wrapper working, you need to setup maven wrapper:

mvn -N io.takari:maven:wrapper

It should create .mvn directory and put needed jar in it.

You're missing the .mvn folder in your git repository. You should have a folder called .mvn which contains the files wrapper/maven-wrapper.jar, wrapper/maven-wrapper.properties and jvm.config. Perhaps you missed it because it's a hidden folder.

Try doing git add -f .mvn from the command line then commit and push.

«I don't know what it is used for»:

A mvnw is a maven wrapper. Basically it is a script that decouples maven from your local installation and may add checks and features to the actual mvn command - for the complete explanation check the official maven documentation ^[1].

To set it up (assuming you had it running solely with mvn) it should suffice to run:

    mvn wrapper:wrapper 

And that should create at least the following new files:

    .
    ├── .mvn
    │   └── wrapper
    │       ├── maven-wrapper.jar
    │       └── maven-wrapper.properties
    ├── mvnw
    └── mvnw.cmd

Then, by evoking mvnw you'd run everything by that maven-wrapper.jar - completely unattached from you local maven installation.

If the maven-wrapper.jar is not in place after the command, you probably have a network issue - the most common one is a proxy that needs to be configured.

«How to fix»:

What is the problem?

Proxy issue:

Initially, the mvn wrapper:wrapper couldn't fetch the maven-wrapper.jar for me.

.
├── .mvn
│   └── wrapper
│       └── maven-wrapper.properties
├── mvnw
└── mvnw.cmd

If you take the deep dive into the rabbit hole and check the actual wrapper code ^[2], you'll notice there is no default proxy - but you can set a proxy for the java virtual machine by creating a .mvn/jvm.config file containing:

    -Dhttp.proxyHost=proxy.domain.name
    -Dhttp.proxyPort=8080
    -Dhttps.proxyHost=proxy.domain.name
    -Dhttps.proxyPort=8080 

With that in place just rerunning the setup command

    mvn wrapper:wrapper

fetched everything by itself:

    .
    ├─ .mvn
    │  ├── jvm.config
    │  └── wrapper
    │      ├── maven-wrapper.jar
    │      └── maven-wrapper.properties
    ├── mvnw
    └── mvnw.cmd

By now, running the ./mvnw clean install, should work ok - it did for me (but only for a few days)

Class Issue:

One nice day, I came into office and the mvnw was returnnig the same error you stated!...

For me the issue was actually a line ending (LF <-> CRLF) issue in the jvm.config file.

As the mvnw script expands the options into an actual command ^[3] the command would get messed up with a CR splitting it before the classpath option. Since that classpath is where the maven-wrapper.jar (containing the MaveWrapperMain) is passed, the java command couldn't actually get the class.

How did I fix it:

I simply re-created the file containing only LFs and no CRLFs.

Bonus:

As I noticed that this CRLF was introduced by a user that was on a windows machine, I checked some git documentation ^[4] to avoid repeating the issue, ran the following and advised everyone to do the same:

    git config --global core.autocrlf true

Hope this centralizes the steps I found scattered and ends up saving time to someone.

1. Adding to what Oleksii Volynskyi,

use the official maven wrapper

mvn wrapper:wrapper

rather than io.takari:maven:wrapper which is no longer maintained

2. As mentioned by Phil Webb add .mvn folder to the git repository

For anyone using Windows seeing this error, check if the .mvn folder files got converted to CRLF. If so, changing them to LF will likely fix your problem.

In my case, I had the project saved in a folder which route included a character with an accent (in my case "á"). Removing the accent fixed the problem.

In my case, there was a .dockerignore file in which you select all the jar files that should be taken into account and the one in .mvn wasn't there, hence the error

i actually did just mvn clean install and it worked, i am actually using older version of spring 2.0 to be specific