How To Fix Error In Confusion Matrix: The Data And Reference Factors Must Have The Same Number Of Levels?

Hello, While I’m working on R caret Linear Regression model. I’m trying to execute a confusion matrix and then I’m getting this below:

Error in confusionMatrix.default(pred, testing$Final): the data and reference factors must have the same number of levels.

Can anyone tell me What does it mean, Why this error occurs, and How to fix this error?