OS Error CURRENT using ArcPy in ArcGIS Pro

I am relatively new in Python.

I have a project that I need to adapt to work with ArcGIS Pro. This code runs in FME PythonCaller using Python 2 working with ArcMap. Now I need to migrate to Python 3 which works with ArcGIS Pro. I had to adapt only the part referring to library ArcPy. Here is a part of the code:

import arcpy
import fme
import fmeobjects

class FeatureProcessor(object):
    def __init__(self):
        pass

    def check_layer_file(self, lyrPath):
        # Collect errors in list
        self.error_messages = []

        # Check whether the layer file is a group layer
        aprx = arcpy.mp.ArcGISProject(r'C:\Path\To\Project\test.lyr')
        map_lyr = aprx.listMaps()[0]
        lyrs = aprx.listLayers(lyrPath)
        glyr = lyrs[0]
        if not glyr.isGroupLayer:
            self.error_messages.append("Message.")

        # Loop over all layers
        for lyr in lyrs:
            if not lyr.isGroupLayer: 

                # Check if all layers have a source
                if lyr.isBroken: # Returns True if a layer's data source is broken.
                    self.error_messages.append(
                        str(lyr) + " has a broken source.")

                # Check if layer is a feature layer
                if lyr.isFeatureLayer:
                    self.error_messages.append(
                        str(lyr) + " is a Feature Layer.")

                # Check for symbology type
                if lyr.symbologyType == "OTHER":
                    self.error_messages.append(str(lyr) + " unsupported.")

        return self.error_messages

Whenever I run it I get an error message: Python Exception: CURRENT.

Python Exception <OSError>: CURRENT
Traceback (most recent call last):
  File "<string>", line 54, in input
  File "<string>", line 15, in check_layer_file
  File "D:\Apps\ArcGIS\Pro\Resources\ArcPy\arcpy\_mp.py", line 344, in __init__
    self._arc_object = arcgisscripting._mapping.ArcGISProject(*gp_fixargs((aprx_path,), True))
OSError: CURRENT

I imagine the code assumes I am running it within ArcGIS Pro with a valid project open, which is not the case.

I have tried pointing directly to the path of the layer but I get the same error only now showing the filepath directory instead of "CURRENT" (Python Exception: filepath example).

Does anyone know what I am doing wrong?