I have OS High Sierra 10.13.4. Can I update to 10.14?

You haven't specified the model of your MacBook Pro. To upgrade to Mojave you will need a Mid 2012 or newer model.

See macOS Mojave - Technical Specifications

The old macOS installers can be found at How to get old versions of macOS. Check your compatibility, make a backup and use Safari to click on the link for macOS Mojave.

Only a suggestion -" I have OS High Sierra 10.13.4. "

The current and last update for High Steer really should be installed to version 10.13.6 and all security Updates ( Last being Nov 2020 ) before attempting the possible UpGrade to Mojave.

UpGrading to a newer version of macOS without being Fully up-to-date on current version - may or may not work as well as you expect.

If your Mac is a 2012 or newer model, or a 2010 desktop Mac Pro upgraded with a Metal-capable graphics card, yes. Otherwise, no.

For reference, that's a full-version upgrade, not an update.

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