The mock location app item is greyed out or the list is empty — what do I do?

There are exactly four reasons, and you can check them in order in a couple of minutes.

The first and most common — there is no suitable app on the system. The “Select mock location app” item opens, but the only option inside is “None”. Android does not list every app here, only those that declared the corresponding capability, so the list stays empty until such an app is installed.

The second — the app is installed, but in another profile. On phones with a work profile or a second space, apps from one profile are not visible to the settings of another. Install it in the profile where you open developer options.

The third — developer options were not fully enabled. Sometimes there were fewer than seven taps on the build number or the system did not ask for the PIN: the section is visible, but some items in it are inactive. Enable it again and make sure the confirmation appears.

The fourth — a device management policy. On work phones with a corporate profile the administrator can forbid mock locations, and the item stays grey whatever you do. The app cannot work around this.

Also check that you are looking at the right item: in older Android versions (before 6.0) instead of choosing an app there was a simple “Allow mock locations” switch, and guides from those years do not apply to modern phones. What this setting is and how it works — in the article what mock location is.