You can't use the crafting or the smithing table to combine the netherite ingot with the diamond tools/armour you have.
Linked issues
Comments 2
Thank you for your report!
We're actually already tracking this issue at MCPE-70218.
Per the 1.16.0.57 patch notes you need to combine the netherite ingot and diamond tool in the smithing table.
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Only smithing table can combine diamond tools + netherite.