As of npm 3.10 you have to use UNLICENSED:

{ "license": "UNLICENSED"}

or

{ "license": "SEE LICENSE IN <filename>"}

The value of license must either one of the options above or the identifier for the license from this list of SPDX licenses. Any other value is not valid.

The following is no longer valid for current versions of npm

For npm versions before 3.10 you may use:

{ "license" : "LicenseRef-LICENSE" }

Then include a LICENSE file at the top level of the package. It could be as short as:

(c) Copyright 2015 person or company, all rights reserved.

But you might want to be more explicit about what is not allowed.

Unfortunately, the format SPDX-License-Identifier: UNLICENSED is an invalid SPDX license expression per the SPDX spec, because the identifier UNLICENSED is not part of the SPDX license list. While well-intentioned, Solidity has here caused potentially dangerous confusion.

The SPDX project is concerned about machine-readable annotations for FOSS licenses (free/libre or open source software). By default, software is “all rights reserved. For such software, it makes no sense to add an SPDX tag. Yet, the Solidity documentation is suggesting to refer to such all-rights reserved software as “unlicensed”.

The Unlicense is a license or license-like legal device that tries to do the very opposite of “all rights reserved”: it is a very permissive license that allows any use, and may even be equivalent to a public domain dedication that relinquishes any copyright. It's a bit confusingly written and has a confusing name so I don't recommend that anyone should use the license, but it's popular with people who don't like to think about licensing and want their software to be used however anyone wants, without limitations.

So Solidity's nonstandard UNLICENSED identifier is VERY DIFFERENT from the SPDX Unlicense identifier. See also Solidity issue #12251. Per SPDX-spec issue #49, it would have been better to represent unlicensed software with NONE, but that's not yet part of the formal SPDX license identifier syntax.

The accepted answer to this question is wrong. The SPDX specification offers LicenseRefs for this use case, so you could put a value such as LicenseRef-acme-corp-proprietary-license in the <name> field, and then a link to the full text of your proprietary license in the <url> field (to allow human lawyers to do appropriate due diligence). Any conforming SPDX processor / expression parser should handle it just fine.

As of SPDX v3.0 (not yet released, at the time of writing), a similar mechanism called AdditionRef will also be offered for License Exceptions, which will make SPDX expressions such as Apache-2.0 WITH AdditionRef-acme-corp-specific-modifications possible.

It's a warning, not an error. So you are able to compile the contract without the license identifier, and it has no effect on it's functionality.

The license identifier is one-line comment in a predefined format that states how can others use (or not use) your source code. List of allowed values linked to the open-source license types: https://spdx.org/licenses/

Examples:

• // SPDX-License-Identifier: MIT states that others can use your source code with respect to the MIT open-source license
• // SPDX-License-Identifier: UNLICENSED states that your code is not open-source

// SPDX-License-Identifier: UNLICENSED
pragma solidity ^0.8;

contract MyContract {
    // your code
}

Docs: https://docs.soliditylang.org/en/v0.8.6/layout-of-source-files.html#spdx-license-identifier

From Solidity ^0.6.8 SPDX license is introduced. So you need to use SPDX-License-Identifier in the code.

Have a look at this: https://forum.openzeppelin.com/t/solidity-0-6-8-introduces-spdx-license-identifiers/2859

For example in your code you need to use license identifier like

// SPDX-License-Identifier: MIT
pragma solidity ^0.7.5;
contract TestContract {
// Some logic
}

You need to use license according to your project. Some other licenses are:

// SPDX-License-Identifier: GPL-3.0-or-later

You can find list of licenses here: https://spdx.org/licenses/