If I run this command normally it works: /execute @p [tag=Visitor] ~1 ~ ~ /say Test ... But if I try to run it in a command block it says: Syntax Error: Unexpected "]": at "xecute @s[>>]<<"
The format that you used is the pre-1.13 command format. Instead of what you did, try this command:
/execute as @p[tag=Visitor] run /say Test
I would also suggest removing the / at the beginning if it is to be place into a command block, as I have run into errors where it doesn't like the /, and disregards the rest of the command.