EA greyed out in MT5 Navigator

Causes, most likely first

1. It is an MT4 file in an MT5 terminal

An .ex4 will never run in MetaTrader 5. The languages and platforms are separate products, not versions of each other, and no conversion tool produces a working result for anything non-trivial. See MT4 vs MT5.

2. The file did not compile

A source file placed in the folder without compiling shows in the tree but cannot be attached. Open MetaEditor and compile it; the errors appear there rather than in the terminal, which is why the terminal gives no explanation.

3. The terminal has not rescanned the folder

Files copied in while MetaTrader is running are not always picked up. Right-click in Navigator and Refresh, or restart the terminal. See EA not in Navigator.

4. It was placed in the wrong directory

Expert Advisors belong in MQL5/Experts, not MQL5/Indicators or MQL5/Scripts. Use File then Open Data Folder rather than navigating to Program Files, which is a different location and a common mistake.

Why this happens

The Experts and Journal tabs tell you which of these it is within seconds, and almost nobody looks at them. A compile failure names the file and the line; a wrong-platform file simply never appears as attachable.

If the EA is a compiled product from a vendor rather than source, a greyed-out entry after a correct install usually means the wrong platform build was downloaded.