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Assuming that `dynamicmodel` and `uncertainlibrary` are just (nested) [dictionaries](https://docs.python.org/3/tutorial/datastructures.html?highlight=dictionary#dictionaries) and 
```python
value = '["SCD"]["CL0"]'
``` 
is actually supposed to be a string (note the single quotes here), your code would attempt to add a dictionary to a string, which is indeed not supported.
Python would let you know by throwing an error of the form:
```text
TypeError: unsupported operand type(s) for +: 'dict' and 'str'
```

If this is indeed your use case (I can't be sure due to missing error message and/or possible typos), then you could probably achieve what you want to do by using more appropriate types to represent the keys for the dictionaries:
```python
value = ("SCD", "CL0")

def generate_val(dynamicmodel, uncertaintylibrary, value):
    key1, key2 = value  # unpack keys in value
    model = dynamicmodel[key1][key2]  # index (nested) dict
    uncertainty = uncertaintylibrary[key1][key2]  # index (nested) dict
    deviation = uncertainty * model  # additional variable
    new_model = np.random.uniform(model - deviation, model + deviation)
    return new_model
```

I chose to make the `value` variable a tuple of the two keys for the nested dictionary indexing.
The parentheses are there for clarity, but they are not necessary (due to tuple packing).
The first line of the function unpacks this tuple to retrieve the two keys for indexing again.
For more information on tuple packing/unpacking, I refer to the [docs](https://docs.python.org/3/tutorial/datastructures.html?highlight=dictionary#tuples-and-sequences).

Note that I took the liberty to take your variables and introduced a new variable to adhere to the [PEP-8](https://peps.python.org/pep-0008/) formatting guidelines (especially variable naming and line length).