Forward slash at the end of the Solr URL caused this issue.

<add name="solr.search" connectionString="https://<solr server domain URL>:8983/solr/" />

After removed the forward slash at the end of URL, working as champ.

Correct way to mention Solr URL in connection string.

<add name="solr.search" connectionString="https://index-lablink.ttgtpmg.net:8983/solr" />

Issue: Indexes are not listed in Indexing Manager.

Error log:

5844 11:31:27 WARN  Error connecting to solr. Solr is not running on the provided url [Url:https://<solr server URL>:8983/solr/]
5844 11:31:27 ERROR Unable to connect to Solr at [https://<solr server URL>:8983/solr/]
Exception: Sitecore.ContentSearch.SolrProvider.SolrConnectors.Exceptions+SolrConnectorException
Message: Unable to connect to Solr at [https://<solr server URL>:8983/solr/]
Source: Sitecore.ContentSearch.SolrProvider
   at Sitecore.ContentSearch.SolrProvider.SolrConnectors.SolrConnector.VerifyConnection(String solrAddress)
   at Sitecore.ContentSearch.SolrProvider.SolrConnectors.SolrConnector.Initialize(String solrAddress)
   at Sitecore.ContentSearch.SolrProvider.SolrSearchIndex.InitializeSolr()

Cause for the issue: Forward slash at the end of the Solr URL caused this issue.

<add name="solr.search" connectionString="https://<solr server domain URL>:8983/solr/" />

Solution:

After removed the forward slash at the end of URL, working as champ.

Correct way to mention Solr URL in connection string.

<add name="solr.search" connectionString="https://index-lablink.ttgtpmg.net:8983/solr" />