Hi, I am 10000000% sure, there was an “Interface”-drop-down in the “Ping-Tool”:
The error-message suggest a “Interface” too, but… well o_O
See this post V7.21.1 [stable] is released! - #27 by CGGXANNX
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Thank you!
But still strange to me, why they make life harder for people and not a bit easier? Why do they remove the drop-down without a hint, a comment or even something useful in the error message? 
Not even here: Ping - RouterOS - MikroTik Documentation
Why? Well, it seems that this is not the only feature that was once easy and intuitive but now requires more inside knowledge. Much like a command line interface.
No idea if this is the direction MikroTik intends to move, but they certainly are.