tl;dr:
With BSD Sed, such as also found on macOS, you must use -i '' instead of just -i (for not creating a backup file) to make your commands work; e.g.:
sed -i '' 's/RABBITMQ_HOST=.*/RABBITMQ_HOST='"$RABBITMQ_HOST"'/' "$Deploy_path"
To make your command work with both GNU and BSD Sed, specify a nonempty option-argument (which creates a backup) and attach it directly to -i:
sed -i'.bak' 's/RABBITMQ_HOST=.*/RABBITMQ_HOST='"$RABBITMQ_HOST"'/' "$Deploy_path" &&
rm "$Deploy_path.bak" # remove unneeded backup copy
Background information, (more) portable solutions, and refinement of your commands can be found below.
Optional Background Information
It sounds like you're using BSD/macOS sed, whose -i option requires an option-argument that specifies the suffix of the backup file to create.
Therefore, it is your sed script that (against your expectations) is interpreted as -i's option-argument (the backup suffix), and your input filename is interpreted as the script, which obviously fails.
By contrast, your commands use GNU sed syntax, where -i can be used by itself to indicate that no backup file of the input file to updated in-place is to be kept.
The equivalent BSD sed option is -i '' - note the technical need to use a separate argument to specify the option-argument '', because it is the empty string (if you used -i'', the shell would simply strip the '' before sed ever sees it: -i'' is effectively the same as just -i).
Sadly, this then won't work with GNU sed, because it only recognizes the option-argument when directly attached to -i, and would interpret the separate '' as a separate argument, namely as the script.
This difference in behavior stems from a fundamentally differing design decision behind the implementation of the -i option and it probably won't go away for reasons of backward compatibility.[1]
If you do not want a backup file created, there is no single -i syntax that works for both BSD and GNU sed.
There are four basic options:
(a) If you know that you'll only be using either GNU or BSD sed, construct the -i option accordingly: -i for GNU sed, -i '' for BSD sed.
(b) Specify a nonempty suffix as -i's option-argument, which, if you attach it directly to the -i option, works with both implementations; e.g., -i'.bak'. While this invariably creates a backup file with suffix .bak, you can just delete it afterward.
(c) Determine at runtime which sed implementation you're dealing with and construct the -i option accordingly.
(d) omit -i (which is not POSIX-compliant) altogether, and use a temporary file that replaces the original on success: sed '...' "$Deploy_path" > tmp.out && mv tmp.out "$Deploy_path".
Note that this is in essence what -i does behind the scenes, which can have unexpected side effects, notably an input file that is a symlink getting replaced with a regular file; -i, does, however, preserve certain attributes of the original file: see the lower half of this answer of mine.
Here's a bash implementation of (c) that also streamlines the original code (single sed invocation with 2 substitutions) and makes it more robust (variables are double-quoted):
#!/bin/bash
RABBITMQ_HOST='rabbitmq1'
RABBITMQ_PASS='12345'
Deploy_path="test.env"
# Construct the Sed-implementation-specific -i option-argument.
# Caveat: The assumption is that if the `sed` is not GNU Sed, it is BSD Sed,
# but there are Sed implementations that don't support -i at all,
# because, as Steven Penny points out, -i is not part of POSIX.
suffixArg=()
sed --version 2>/dev/null | grep -q GNU || suffixArg=( '' )
sed -i "${suffixArg[@]}" '
s/^\(RABBITMQ_HOST\)=.*/\1='"$RABBITMQ_HOST"'/
s/^\(RABBITMQ_PASS\)=.*/\1='"$RABBITMQ_PASS"'/
' "$Deploy_path"
Note that with the specific values defined above for $RABBITMQ_HOST and $RABBITMQ_PASS, it is safe to splice them directly into the sed script, but if the values contained instances of &, /, \, or newlines, prior escaping would be required so as not to break the sed command.
See this answer of mine for how to perform generic pre-escaping, but you may also consider other tools at that point, such as awk and perl.
[1] GNU Sed considers the option-argument to -i optional, whereas BSD Sed considers it mandatory, which is also reflected in the syntax specs. in the respective man pages: GNU Sed: -i[SUFFIX] vs. BSD Sed -i extension.
