Namecheap - Squarespace - Mailchimp

I bought my domain name from name cheap 5 days ago and linked it to Squarespace per the Squarespace instructions - I then registered by a google workspace account to use for an email address. 

Following Squarespace and google instructions I needed to change/update MX so I went make to name cheap to do that. Could not, so I had to revert the name servers to set up gmail. 

Also received the following from their support team: 

According to our check, the domain aceratravel.com is pointed to the third-party nameservers:

Name Server: CONNECT1.SQUARESPACEDNS.COM
Name Server: CONNECT2.SQUARESPACEDNS.COM

Advanced DNS manager is not available for your domain since you have delegated it to custom nameservers . This means that DNS zone and all DNS host records (including A/CNAME/MX/TXT records) are to be managed from the side of third party nameservers. You might need to contact your current DNS/hosting provider to check if it’s possible to modify host records on their end.

I'm receiving test emails to the gmail I set up BUT I cannot link a new Mailchimp account. I can send emails out and I am receiving them.

Mailchimp CS sent this:

Reviewing our logs, we can see that activation emails sent to the address "alexandra@aceratravel.com   " are bouncing with the following message from the receiving server:
 
554 5.7.1 <alexandra@aceratravel.com>: Relay access denied

I've whitelisted Mailchimp - should be all set. Still cannot authenticate my account. 

I'm not sure what to do here and if my gmail will stop working if I change back the name servers to Squarespace. Currently my site is still working but TBD.

Any guidance would be helpful!