Transaction &1 is locked (locked development object)

What causes this issue?

Transaction cannot be started in this logical system because the
transaction was locked.
A transaction can either be locked locally by the system administrator
(customizing),
or can be delivered locked by the application developer.

System Response

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

How to fix this error?

If you need to use the transaction despite the lock, contact your system
adminstrator.

Procedure for System Administrators

Changes to this type of application start locks are regarded as
modifications. They are maintained in transaction SM01_DEV.
You can find more information in the application documentation or in SAP
Note 2234192.

Error message extract from SAP system. Copyright SAP SE.