Editing field in .gdb feature class

[RahulPawar1]

I am adding a text/string field, "<Null>", to a .gdb feature class table.
When I use Field Calculator to change, I get this error message
The calculated value is invalid for the row with ObjectID = 16666.
For example, the calculated value may be too large for the field or you may be trying to add a string to a number field.
This row will not be updated. Do you want to process the rest of the rows?

[DanPatterson]

What was the field calculation, and what was the type of the field you created? 

The message indicates that the field type isn't appropriate for the number you are trying to put into it.

For instance "short integer"

ArcGIS field data types—ArcGIS Pro | Documentation

So compare your expected calculation result with the data types for fields

[RahulPawar1]

I want to put Null in the all field instead of unknown...

[RahulPawar1]

 

[RahulPawar1]

 

[DanPatterson]

For your field calculation use 

None

which will give rise to a warning... which is fine

003571: Certain rows set to NULL due to error while evaluating Python expression: Cannot be evaluated to value.
Description
A failure occurred when attempting to write a calculated value into the target field. This may occur if the value type is incompatible with the target field type. For example, the Python code returned a string value of A, but the target field is of type long.

A null value will be stored in the target field for that row.

Solution
Review the target field and identify records with null values.

[RahulPawar1]

Hey Dan, Thanks Man for the prompt solution. Will try this and let you know.😊

[JohannesLindner]

<Null> is ArcGIS's way of showing that there is no value in a field.

What you were trying to do is put the string "<Null>" (which is absolutely not the same as null) into an integer field. ArcGIS can't convert that string into a number, so it errors out.

To remove a value from a field, you have to use the null value of the language you're using. None for Python (as Dan told you), Null for VB Script, null for Arcade.

[SLouq]

Why Python has to be so different? LOL 😊