# Step 1: Error 1

## Error Message

```{code-block} console
---
caption: Error message 1 when running Step 1.
linenos: True
emphasize-lines: 15,16
---
$ pylith step01_gravity.cfg

Traceback (most recent call last):
  File "/software/baagaard/py38-venv/pylith-debug/bin/pylith", line 28, in <module>
    start(applicationClass=PyLithApp)
  File "/software/baagaard/py38-venv/pylith-debug/lib/python3.8/site-packages/pythia/pyre/applications/__init__.py", line 42, in start
    shell.run(**kwds)

  # -- many line omitted --

  File "/software/baagaard/py38-venv/pylith-debug/lib/python3.8/site-packages/pythia/pyre/inventory/ConfigContext.py", line 68, in configureComponent
    component._validate(self)
  File "/software/baagaard/py38-venv/pylith-debug/lib/python3.8/site-packages/pylith/bc/DirichletTimeDependent.py", line 120, in _validate
    raise ValueError(f"No constrained degrees of freedom found for time-dependent Dirichlet boundary condition '{self.aliases[-1]}'. "
ValueError: No constrained degrees of freedom found for time-dependent Dirichlet boundary condition 'bc_xpos'.
'constrained_dof' must be a zero-based integer array (0=x, 1=y, 2=z).
```

## Troubleshooting Strategy

The full error message includes a Python Traceback followed by the actual error message (lines 15-16) generated by the `ValueError` exception.
The error message indicates there is a problem with the parameters for `pylithapp.problem.bc.bc_xpos`.
We look for this section in the parameter files and find it in `pylithapp.cfg`.

## Resolution

There is a typo in the section heading for `pylithapp.problem.bc.bc_xpos`.
Because the section heading has the wrong name, PyLith uses the defaults (no constrained degrees of freedom) and complains that no degrees of freedom are constrained by the Dirichlet boundary condition.

```{code-block} cfg
---
caption: Correct error in `pylithapp.cfg`.
---
# Error
[pylithapp.timedependent.bc.xpos]

# Correct
[pylithapp.timedependent.bc.bc_xpos]
```