Can I convert to Roth without tax consequences?

I retired in 2021, rolling over my 401K to an IRA.  I also have a separate investment account that was always paid into with post-tax dollars and I have paid taxes on dividends every year.  If I take money out of that account, is there a tax consequence?  Would there be a tax consequence if I converted it to a Roth first...or can I even convert it without a tax consequence?  Thanks for your help.