Check that there are no odd characters in the tab name(s) of the sheet. 

Can also try moving the data to a fresh excel with copy paste to make sure it,s nothing to do with that particular file.

A few things to try: 

1. Learn more about Org charts and importing the data in Help files
2. Start with a data csv template provided by Lucid
3. Save a copy of your data as a csv file and import CSV instead of XLSX (only 1 tab in CSV)

Good suggestion on the strange characters ​@Paul McC, it’s a thing  

Couldn’t post with my shortcut URLs so I’m trying to just link full URLs below, hoping this works: 

LucidChart screenshots: https://lucid.app/lucidchart/d8f8b5b2-a5e4-4df7-8ff4-59b4a1c1cfc0/edit?viewport_loc=-394%2C-177%2C1964%2C1247%2C0_0&invitationId=inv_39861dac-723c-426f-9664-91cd7eb2de0c 

1. Org Chart Help File: www.lucidchart.com/pages/how-to-make-an-org-chart

2. CSV Template Download: https://cdn-cashy-static-assets.lucidchart.com/app/chart/plugins/orgchart2018/org-chart-example.csv

Thanks good advices as well - but also got an advice to open in an incognito sheet and that solved the problem - Really strange I have worked a lot in Lucid in my previous company but never had that issue before but incognito mode helped.

Thanks for sharing the solution that worked for you!!!