I am trying to attach a file in my email. But it is not allowing me to?

I presume you are using gmail. If you have a file stored in Google Drive, follow steps 1 through 4. If you have the file on your PC, seem the bold notation in step 4.

Step 1: Open Gmail in your browser, then click Compose to create a new message.
Step 2: Near the bottom of the new-mail window, locate and click the Google Drive icon.
Step 3: Now you'll see the contents of your Google Drive. Choose one or more files, then select the Attachment option in the bottom-right corner. At first glance it may not look like anything has happened, but basically you're just toggling between that and the Drive Link option.
Step 4: Click the blue Insert button (bottom-left corner), then send your message as usual.
This is a bit different than adding an attachment that's stored on your PC, which is done by clicking the paperclip icon (in step 2) instead.

Most email servers refuse to send email attachments above 10 MB. Email was never designed to be storage for files.