Since the PSK (Pre-Shared Key) is masked, we are unable to see if the key is being cut off due to too many characters.
Therefore, knowing the maximum key length is helpful.
In R80.x SmartConsole:
VPN Community properties -> Shared Secret:

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In R77X SmartDashboard:
VPN Community properties -> Advanced Settings -> Shared Secret:

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If the PSK (Pre-Shared Key) is too short, or too long, an alert will pop up saying the following:
"The secret must be at least six characters long, no more than 64 characters, and contain four different characters"