iPad Pro 12.9” Gen 1 on iPadOS 13.5.1 with Minecraft 1.16.0.
/locatebiome wasn’t added in the 1.16.0 update. This is a parity issue as it is a command added in 1.16.0 Java.
As of May 4th parity issues were permitted to be reported as of this publication. https://feedback.minecraft.net/hc/en-us/articles/360015877192-Parity-Requests-and-You-A-Guide