Thing is, the specific rule of Manifest Echo allows you only to make an attack of opportunity. If you cast a spell it is not classifying for the reaction granted by Manifest Echo anymore.

• When a creature that you can see within 5 feet of your echo moves at least 5 feet away from it, you can use your reaction to make an opportunity attack against that creature as if you were in the echo’s space.

Those are two different specific rules. In the end, the DM decides which one of those specific rules counts.

Specific rules don't always have to allow something, they can also disallow something by stating what the only things are that can be done.

Thing is, the specific rule of Manifest Echo allows you only to make an opportunity attack. If you cast a spell it is not classifying for the reaction granted by Manifest Echo anymore.

When a creature that you can see within 5 feet of your echo moves at least 5 feet away from it, you can use your reaction to make an opportunity attack against that creature as if you were in the echo’s space.

Those are two different specific rules. In the end, the DM decides which one of those specific rules counts.

Specific rules don't always have to allow something, they can also disallow something by stating what the only things are that can be done.