The Steps to resolve this issue with FusionAuth & Zoom SSO
Tested with FusionAuth Version: 1.19+

Before FusionAuth, we would need Few settings from Zoom SSO

1. Login to Zoom Account > My Account > Advanced > Single Sign-On > Enable
2. Copy: Service Provider (SP) Entity ID setting, it should be either <vanity>.zoom.us or https://<vanity>.zoom.us/
3. You will need this for configuring Fusion Auth, (Yes, Zoom will not allow to save SAML until all the fields are filled, don't worry, we will come there later. keep zoom settings page open)

Now, lets create & configure FusionAuth App for zoom sso

1. Login to FusionAuth with Admin Access
2. Goto > Application > Create New Application As usual with Default Configuration.

Note: Before Configuring SAML Settings in FusionAuth, we need to create

• SHA-256 Certificate with Proper Issuer required by Zoom
• An appropriate Lambda Function to match the Response expected by Zoom
  Follow these steps for both of this

CREATE SHA-256 CERTIFICATE FOR ZOOM

1. Go To > Settings > Key Master
2. "Generate RSA" From Top Right Drop Button

Name: Any Name, its for Identification, e.g. ZoomSAMLCertificateKey
Issuer: <vanity>.zoom.us (Should match the value set in Zoom's SAML "Service Provider (SP) Entity ID" setting)
Algorithm: RSA using SHA-256
Key lenght: 2048

3. Submit
   ==============================

CREATE SAML Populate Lambda as Required by Zoom

1. Go To > Customizations >Lambda > Add
2. Create New Lambda from top right [+] button

Name: Any Name for Identification: e.g. "SAML v2 Populate Lambda for Zoom App"
Type: SAML v2 Populate
Debug Enabled: as required

Body:

        function populate(samlResponse, user, registration) {
                    samlResponse.assertion.subject.subjectConfirmation.notBefore = null;
                    samlResponse.assertion.conditions.notBefore = null;
        }

3. Save
   ==============================

Now, we are ready to configure SAML settings in our App

5. Go To > Applications > Newly Created App > Enable SAML
6. Configure SAML Settings as following

Issuer: <vanity>.zoom.us (Should match the value set in Zoom's SAML "Service Provider (SP) Entity ID" setting)
Audience: leave it blank (default)
Callback URL (ACS): https://<vanity>.zoom.us/saml/SSO
Logout URL: https://<vanity>.zoom.us/ (or where ever to redirect after logout)
Signing key: Select the Key Generated in previous step e.g. "ZoomSAMLCertificateKey"
XML signature canonicalization method: Exclusive
Response populate lambda: Recently Created Lambda e.g. "SAML v2 Populate Lambda for Zoom App"
Debug Enabled: as required

Done, with FusionAuth, its ready for Zoom SSO

Now come to Zoom Page and Copy required settings from FusionAuth

1. You will have most details from the FusionAuth Application
2. Go to > Applications List > Click on our newly created Zoom App
3. Scroll to "SAML v2 Integration details" section

Get Zoom's SAML Settings from FusionAuth

Zoom's Sign-in Page URL:     <---     FA's Login URL
Zoom's Sign-out Page URL:     <---     FA's Logout URL
Zoom's Service Provider (SP) Entity ID == Select whatever you choose earlier as Issuer during Certificate Creation
Zoom's Issuer (IDP Entity ID):     <---     FA's Entity Id

Zoom's Identity Provider Certificate:     <---     
GoTo > FusionAuth's Settings > Key-Master > Click 🔍 on our Key generated for Zoom App
the value in "Base64 encoded" is to be used for Zoom's Identity Provider Certificate	

Zoom's Binding: HTTP-Redirect
Zoom's Signature Hash Algorithm: SHA-256
Zoom's Security: 
      Sign SAML request -- Unchecked
      Sign SAML Logout request -- Unchecked
      Support encrypted assertions -- Unchecked
      Enforce automatic logout after user has been logged in for -- Unchecked
      Save SAML response logs on user sign-in -- As Required
Zoom's Provision User: At Sign-in (Default) or As Required

4. [Save Changes] in Zoom
5. It's DONE! It should work as intended.
   Note: there can be errors still, but mostly will not be related to SAML.

Zoom engineering team tried is also ready and trying its best to support FusionAuth. They said me if we figure this out it will be a support FusionAuth officially.

SAML is very complicated, I would recommend purchasing a paid support plan to get additional engineering assistance as you are doing with Zoom. This is likely a signing configuration error.

After a lot of working out, they said, it is probably failing due to "NotBefore" attribute in the Assertion>Conditions tag.

This was fixed in 1.28.0.
https://github.com/FusionAuth/fusionauth-issues/issues/1215

Zoom Error Message Says:
The signature is not trusted or invalid, please check the certificate.

Also, I could figure out to remove the tags and NotBefore attribute using lambda but still, the problem persists.

Zoom engineering team tried is also ready and trying its best to support FusionAuth. They said me if we figure this out it will be a support FusionAuth officially.

My ticket is still on.

Kindly help